hello_att.s

.text
.globl _start

_start:
    mov $1, %rax
    mov $1, %rdi
    mov $msg, %rsi
    mov $14, %rdx
    syscall

    mov $60, %rax
    mov $0, %rdi
    syscall

.data
msg:
    .ascii "Hello, world!\n"

hello_intel.s

.intel_syntax
.text
.globl _start

_start:
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    mov rdx, 14
    syscall

    mov rax, 60
    mov rdi, 0
    syscall

.data
msg:
    .ascii "Hello, world!\n"

output.txt

[nix-shell:~/tmp]$ $(CC) -o hello_intel hello_intel.s  -nostdlib && ./hello_intel

[nix-shell:~/tmp]$ $(CC) -o hello_att hello_att.s  -nostdlib && ./hello_att
Hello, world!

[nix-shell:~/tmp]$ objdump -d hello_intel -s -M intel

hello_intel:     file format elf64-x86-64

Contents of section .text:
 4000e8 48c7c001 00000048 c7c70100 0000488b  H......H......H.
 4000f8 34251701 600048c7 c20e0000 000f0548  4%..`.H........H
 400108 c7c03c00 000048c7 c7000000 000f05    ..<...H........ 
Contents of section .data:
 600117 48656c6c 6f2c2077 6f726c64 210a      Hello, world!.  

Disassembly of section .text:

00000000004000e8 <_start>:
  4000e8:	48 c7 c0 01 00 00 00 	mov    rax,0x1
  4000ef:	48 c7 c7 01 00 00 00 	mov    rdi,0x1
  4000f6:	48 8b 34 25 17 01 60 	mov    rsi,QWORD PTR ds:0x600117
  4000fd:	00 
  4000fe:	48 c7 c2 0e 00 00 00 	mov    rdx,0xe
  400105:	0f 05                	syscall 
  400107:	48 c7 c0 3c 00 00 00 	mov    rax,0x3c
  40010e:	48 c7 c7 00 00 00 00 	mov    rdi,0x0
  400115:	0f 05                	syscall 

[nix-shell:~/tmp]$ objdump -d hello_att -s -M intel

hello_att:     file format elf64-x86-64

Contents of section .text:
 4000e8 48c7c001 00000048 c7c70100 000048c7  H......H......H.
 4000f8 c6160160 0048c7c2 0e000000 0f0548c7  ...`.H........H.
 400108 c03c0000 0048c7c7 00000000 0f05      .<...H........  
Contents of section .data:
 600116 48656c6c 6f2c2077 6f726c64 210a      Hello, world!.  

Disassembly of section .text:

00000000004000e8 <_start>:
  4000e8:	48 c7 c0 01 00 00 00 	mov    rax,0x1
  4000ef:	48 c7 c7 01 00 00 00 	mov    rdi,0x1
  4000f6:	48 c7 c6 16 01 60 00 	mov    rsi,0x600116
  4000fd:	48 c7 c2 0e 00 00 00 	mov    rdx,0xe
  400104:	0f 05                	syscall 
  400106:	48 c7 c0 3c 00 00 00 	mov    rax,0x3c
  40010d:	48 c7 c7 00 00 00 00 	mov    rdi,0x0
  400114:	0f 05                	syscall