Put build_config.rb into mruby root dir, make, and put build_framework.rb into build dir and run it to get a MRuby.framework, run collect_mrb_gem_archive.rb <gem_name> in build dir to get a seperate <gem_name>.framework

build_config.rb

MRuby::Build.new do |conf|
  toolchain :clang
  conf.gembox 'default'
end

def crossbuild_for(name, platform, sysroot, cc_defines = [])
  MRuby::CrossBuild.new(name) do |conf|
    toolchain :clang

    conf.gembox 'default'
    conf.bins = []

    conf.cc do |cc|
      cc.command = 'xcrun'
      cc.defines = cc_defines
      cc.flags = %W(-sdk iphoneos clang -miphoneos-version-min=7.0 -arch #{platform} -isysroot #{sysroot} -g -Ofast -Wall -Werror-implicit-function-declaration -fembed-bitcode)
    end
    conf.linker do |linker|
      linker.command = 'xcrun'
      linker.flags = %W(-sdk iphoneos clang -miphoneos-version-min=7.0 -arch #{platform} -isysroot #{sysroot})
    end
  end
end

SIM_SYSROOT = %x[xcrun --sdk iphonesimulator --show-sdk-path].strip
DEVICE_SYSROOT = %x[xcrun --sdk iphoneos --show-sdk-path].strip
crossbuild_for('ios-arm64', 'arm64', DEVICE_SYSROOT, %w(MRB_INT64))
crossbuild_for('ios-armv7', 'armv7', DEVICE_SYSROOT, %w(MRB_INT64))
crossbuild_for('ios-simulator-x86_64', 'x86_64', SIM_SYSROOT, %w(MRB_INT64))
crossbuild_for('ios-simulator-i386', 'i386', SIM_SYSROOT, %w(MRB_INT64))

build_framework.rb

#!/usr/bin/env ruby
require "FileUtils"

BUILD_PATH = File.dirname(__FILE__)

def build_framework(name, lib_files, header_dir)
  path = "#{BUILD_PATH}/#{name}.framework"

  FileUtils.rm_rf path

  FileUtils.mkdir_p "#{path}/Headers"
  FileUtils.cp_r "#{header_dir}/.", "#{path}/Headers"

  Dir.glob("#{path}/Headers/**/*.h").each do |file|
    replaced = File.read(file).gsub(/^#include "mruby\/(.+)"$/, '#include <Mruby/mruby/\1>')
      .gsub(/^#include "mruby\.h"$/, '#include <MRuby/mruby.h>')
    File.open(file, "w") { |f| f.puts replaced }
  end

  File.open "#{path}/Headers/mruby-umbrella.h", "w" do |file|
    file.puts '#define MRB_INT64'
    file.puts '#include "mruby.h"'
    Dir.chdir "#{path}/Headers" do
      Dir["mruby/*.h"].each do |f|
        next if f == "mruby/debug.h"
        next if f =~ /mruby\/boxing/
        file.puts "#include \"#{f}\""
      end
    end
  end

  Dir.mkdir "#{path}/Modules"
  File.open "#{path}/Modules/module.modulemap", "w" do |file|
    file.write <<EOF
framework module MRuby {
  umbrella header "mruby-umbrella.h"

  exclude header "mruby/boxing_nan.h"
  exclude header "mruby/boxing_no.h"
  exclude header "mruby/boxing_word.h"
  exclude header "mruby/debug.h"

  export *
  module * { export * }
}
EOF
  end

  system "lipo #{lib_files.join " "} -create -output #{path}/#{name}"
end

lib_files = %w(arm64 armv7 simulator-x86_64 simulator-i386).map do |arch|
  "#{BUILD_PATH}/ios-#{arch}/lib/libmruby.a"
end
header_dir = "#{BUILD_PATH}/../include"

build_framework "MRuby", lib_files, header_dir

collect_mrb_gem_archive.rb

#!/usr/bin/env ruby

require 'tmpdir'
require 'fileutils'

def collect_gem gem_name, dir, subdir, arch
  object_files = Dir.glob("./#{dir}/#{subdir}/mrbgems/#{gem_name}/**/*.o").map do |path|
    File.absolute_path path
  end

  gem_funcname = File.read("./#{dir}/#{subdir}/mrbgems/#{gem_name}/gem_init.c").split("\n").select do |line|
    line =~ /GENERATED_TMP_mrb_(.+)_gem_init/
  end.first.match(/GENERATED_TMP_mrb_(.+)_gem_init/)[1]

  init_func_name = "GENERATED_TMP_mrb_#{gem_funcname}_gem_init"
  final_func_name = "GENERATED_TMP_mrb_#{gem_funcname}_gem_final"

  header_content = <<-EOF
#include <MRuby/mruby.h>

void mrbgem_load_#{gem_funcname}(mrb_state *mrb);
  EOF

  source_content = <<-EOF
typedef void (*mrb_atexit_func)(void*);
extern void mrb_state_atexit(void *mrb, mrb_atexit_func func);
extern void #{init_func_name}(void *mrb);
extern void #{final_func_name}(void *mrb);

__attribute__((visibility("default")))
void mrbgem_load_#{gem_funcname}(void *mrb) {
  #{init_func_name}(mrb);
  mrb_state_atexit(mrb, #{final_func_name});
}
  EOF

  sdk_path = arch =~ /arm/ ? %x[xcrun --sdk iphoneos --show-sdk-path].strip : %x[xcrun --sdk iphonesimulator --show-sdk-path].strip

  dir = Dir.mktmpdir "mrbgem_#{gem_funcname}"
  at_exit do
    FileUtils.rm_rf dir
  end

  Dir.chdir dir do
    source_file_path = "init.c"
    extra_object_file_path = "init.o"

    File.open source_file_path, "w" do |f|
      f.write source_content
    end

    system "xcrun", "clang", "-g", "-miphoneos-version-min=7.0", "-fembed-bitcode", "-Os", "-arch", arch, "-isysroot", sdk_path, "-c", source_file_path, "-o", extra_object_file_path

    object_files << extra_object_file_path

    output_a_file_path = "#{gem_funcname}.a"
    system "libtool", "-static", "-syslibroot", sdk_path, *object_files, "-o", output_a_file_path

    output_h_file_path = "#{gem_funcname}.h"
    File.open output_h_file_path, "w" do |f|
      f.write header_content
    end

    [File.absolute_path(output_a_file_path), File.absolute_path(output_h_file_path)]

  end
end

GEM_NAME = ARGV[0] || 'mruby-dispatch'

outputs = [
  collect_gem(GEM_NAME, '.', 'ios-arm64', 'arm64'),
  collect_gem(GEM_NAME, '.', 'ios-armv7', 'armv7'),
  collect_gem(GEM_NAME, '.', 'ios-simulator-x86_64', 'x86_64'),
  collect_gem(GEM_NAME, '.', 'ios-simulator-i386', 'i386'),
]

header_file_path = outputs.first.last # header files are all the same
lib_files = outputs.map(&:first)

framework_path = "#{GEM_NAME}.framework"
FileUtils.rm_rf framework_path if Dir.exist? framework_path
FileUtils.mkdir_p "#{framework_path}/Headers"

system "lipo", *lib_files, "-create", "-output", "#{framework_path}/#{GEM_NAME}"
FileUtils.cp header_file_path, "#{framework_path}/Headers/#{GEM_NAME}.h"

puts "-> #{framework_path}"