autoconf: Unify CFLAGS_SSE42 and CFLAGS_ARMV8_CRC32C

autoconf: Unify CFLAGS_SSE42 and CFLAGS_ARMV8_CRC32C

Until now we emitted the cflags to build the CRC objects into architecture
specific variables. That doesn't make a whole lot of sense to me - we're never
going to target x86 and arm at the same time, so they don't need to be
separate variables.

It might be better to instead continue to have CFLAGS_SSE42 /
CFLAGS_ARMV8_CRC32C be computed by PGAC_ARMV8_CRC32C_INTRINSICS /
PGAC_SSE42_CRC32_INTRINSICS and then set CFLAGS_CRC based on those. But it
seems unlikely that we'd need other sets of CRC specific flags for those two
architectures at the same time.

This simplifies the upcoming meson PGXS compatibility.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/20221005200710.luvw5evhwf6clig6@awork3.anarazel.de