0static.sh

cc -nostdlib hello.c syscall.S -static -no-pie -lc

1dynamic.sh

cc -nostdlib -o /usr/lib/libc.so.0 syscall.S -shared -Wl,--soname=libc.so.0
rm libcx.so
ln -s /usr/lib/libc.so.0 libcx.so
cc -nostdlib hello.c -L . -lcx -Wl,-Bstatic -lc

hello.c

#include <string.h>
#include <sys/syscall.h>

long syscall(int, ...);

void
_start(void) {
	void *p = __builtin_frame_address(0);
	int argc = ((int*)p)[2];
	char **argv = &((char**)p)[2];
	static char msg[64] = "Hello, ";
	strcat(msg, argc > 1 ? argv[1] : "world");
	strcat(msg, "!\n");
	syscall(SYS_write, 1, msg, strlen(msg));
	syscall(SYS_exit, 0);
}

#if __PIC__
asm (
	".pushsection .note.openbsd.ident, \"a\"\n"
	".long 8, 4, 1\n"
	".string \"OpenBSD\"\n"
	".long 0\n"
	".popsection\n"
);
#endif

syscall.S

.globl errno
.globl syscall

.bss
errno:
	.long 0

.text
syscall:
	mov %rdi, %rax
	mov %rsi, %rdi
	mov %rdx, %rsi
	mov %rcx, %rdx
	mov %r8, %r10
	mov %r9, %r8
	mov 8(%rsp), %r9
99:	syscall
	jnc 1f
#if __PIC__
	mov errno@GOTPCREL(%rip), %rdi
	mov %eax, (%rdi)
#else
	mov %eax, errno
#endif
	mov $-1, %eax
1:	ret

.section .openbsd.syscalls, ""
.long 99b, 1
.long 99b, 4

.section .note.openbsd.ident, "a"
.long 8, 4, 1
.string "OpenBSD"
.long 0