Kernel fails to execute a.out

Hi, I'm having problems running a program I wrote.
I wrote this program that should just exit with exitcode 44:

// prog.S
#include <sys/syscall.h>
.text
.globl _start
_start:
subl $8, %esp
pushl $44
movl $SYS_exit, %eax
pushl $0
int $0x80

I compiled with

$ cc prog.S -nostdlib -o a.out

and run

$./a.out

Doing so on FreeBSD 13.0-RELEASE i386 (clang 11.0.1) worked fine.
In fact, the executable runs and the exit code of the program is 44
as it should be.

However, doing the same on OpenBSD 7.0 GENERIC.MP#5 i386
and on NetBSD 9.2 i386 (gcc 7.5.0), the kernel refused to execute the
code and it was passed to the shell, which of course failed:

openbsd$ ./a.out ./a.out[1]: syntax error: `(' unexpected

openbsd$ file a.out

a.out: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
statically linked, not stripped

openbsd$ objdump -d a.out

a.out: file format elf32-i386

Disassembly of section .text:

00001184 <_start>:
1184: 83 ec 08 sub $0x8,%esp
1187: 6a 2c push $0x2c
1189: b8 01 00 00 00 mov $0x1,%eax
118e: 6a 00 push $0x0
1190: cd 80 int $0x80


I saw, however, that changing _start to main and compiling without
-nostdlib worked fine.
What am I doing wrong?