hello world in assembler under x86-64 and ARMv7-A Linux

hello-arm.lst

$ gdbdis -r hello-arm _start
   0x00010098 <+0>:	04 27	movs	r7, #4
   0x0001009a <+2>:	01 20	movs	r0, #1
   0x0001009c <+4>:	03 49	ldr	r1, [pc, #12]	; (0x100ac <_start+20>)
   0x0001009e <+6>:	5f f0 0d 02	movs.w	r2, #13
   0x000100a2 <+10>:	00 df	svc	0
   0x000100a4 <+12>:	f8 27	movs	r7, #248	; 0xf8
   0x000100a6 <+14>:	00 20	movs	r0, #0
   0x000100a8 <+16>:	00 df	svc	0
   0x000100aa <+18>:	00 00	movs	r0, r0
   0x000100ac <+20>:	b0 00 01 00	strheq	r0, [r1], -r0	; <UNPREDICTABLE>

hello-arm.S

	.syntax unified
	.thumb

#include <sys/syscall.h>

	.section .text
	.global _start
	.thumb_func
_start:
	/* write(1, "hello, world", 13) */
	movs r7, SYS_write
	movs r0, 1
	ldr r1, =hello_str
	movs r2, hello_str_len
	svc 0

	/* exit_group(0) */
	movs r7, SYS_exit_group
	movs r0, 0
	svc 0

	.section .rodata
hello_str:
	.ascii "hello, world\n"
	.set hello_str_len, . - hello_str

hello-x86-64.lst

$ gdbdis -r hello-x86-64 _start
   0x00000000004000d4 <+0>:	48 c7 c0 01 00 00 00	mov    $0x1,%rax
   0x00000000004000db <+7>:	48 c7 c7 01 00 00 00	mov    $0x1,%rdi
   0x00000000004000e2 <+14>:	48 c7 c6 02 01 40 00	mov    $0x400102,%rsi
   0x00000000004000e9 <+21>:	48 c7 c2 0d 00 00 00	mov    $0xd,%rdx
   0x00000000004000f0 <+28>:	0f 05	syscall 
   0x00000000004000f2 <+30>:	48 c7 c0 e7 00 00 00	mov    $0xe7,%rax
   0x00000000004000f9 <+37>:	48 c7 c7 00 00 00 00	mov    $0x0,%rdi
   0x0000000000400100 <+44>:	0f 05	syscall 

hello-x86-64.S

#include <sys/syscall.h>

	.section .text
	.global _start
_start:
	/* write(1, "hello, world\n", 13) */
	mov $SYS_write, %rax
	mov $1, %rdi
	mov $hello_str, %rsi
	mov $hello_str_len, %rdx
	syscall

	/* exit_group(0) */
	mov $SYS_exit_group, %rax
	mov $0, %rdi
	syscall

	.section .rodata
hello_str:
	.ascii "hello, world\n"
	.set hello_str_len, . - hello_str