Rename pg_crc32c_sse42_choose.c for general purpose

Rename pg_crc32c_sse42_choose.c for general purpose

Future commits will consolidate the CPU feature detection functionality
now scattered around in various files, and the CRC "*_choose.c"
files seem to be the natural place for it. For now, just rename in
a separate commit to make it easier to follow the git log. Do the
minimum necessary to keep the build systems functional, and build the
new file pg_cpu_x86.c unconditionally using guards to control the
visibility of its contents, following the model of some more recent
files in src/port.

Limit scope to x86 to reduce the number of moving parts, since the
motivation for doing this now is to clear out some technical debt
before adding AVX2 detection. Arm is left for future work.

Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/CANWCAZbgEUFw7LuYSVeJ=Tj98R5HoOB1Ffeqk3aLvbw5rU5NTw@mail.gmail.com