Fix unportable use of __builtin_constant_p

Fix unportable use of __builtin_constant_p

On MSVC Arm, USE_ARMV8_CRC32C is defined, but __builtin_constant_p
is not available. Use pg_integer_constant_p and add appropriate
guards. There is a similar potential hazard for the x86 path, but
for now let's get the buildfarm green.

Oversight in commit fbc57f2bc, per buildfarm member hoatzin.