doesn't support atomic_store api

[yarshure]

`root@freebsd:~ # cat test_atomic.c
#include <stdatomic.h>

struct {
atomic_int a;
} foo;

struct {
atomic_int a;

} attribute((packed)) bar;

int main(void)
{
foo.a = 42;
bar.a = 42;
return 0;
}
root@freebsd:~ # cc test_atomic.c
/usr/bin/ld: error: undefined symbol: __atomic_store

referenced by test_atomic.c
/tmp/test_atomic-c67b0b.o:(main)
cc: error: linker command failed with exit code 1 (use -v to see invocation)
root@freebsd:~ # uname -a
FreeBSD freebsd 12.0-RELEASE FreeBSD 12.0-RELEASE r341666 GENERIC amd64`