Portability: check 64-bit GNU atomic operators are useable

Portability: check 64-bit GNU atomic operators are useable

Sometimes (namely 32-bit OpenBSD libstdc++) do not fully implement the
GNU atomic operators for both 32-bit and 64-bit. But Squid makes use of
both types if the compiler deems them required.

We need to check them all before declaring the atomics usable, or not.

 Thanks to Stuart Henderson for identifying the issue.