atomic: Add support for gcc __sync intrinsics

[Lastique]

This adds support for older gcc versions that didn't support __atomic intrinsics.

Related to #446 (comment)

CC @cspiel1

[cspiel1]

Thanks! I will try this with our blackfin build system.

[cspiel1]

src/main/main.c: In function 're_main':
src/main/main.c:1070: error: expected ')' before ';' token
src/main/main.c:1094: error: expected ')' before ';' token

Something wrong here with this line:

	if (re_atomic_rlx(&re->polling)) {

[cspiel1]

Looks good now. It builds. Thanks!