Re: Use __attribute__((target(sse4.2))) for SSE42 CRC32C

- prog = '''
+ sse42_crc_prog = '''

nitpick: Why does this need to be renamed?

+#ifdef TEST_SSE42_WITH_ATTRIBUTE
+#if defined(__has_attribute) && __has_attribute (target)
+__attribute__((target("sse4.2")))
+#endif
+#endif

So, for meson builds, we do test with and without
__attribute___((target(..."))), but for autoconf builds, we check for
__SSE4_2__ to determine whether we need a runtime check. This difference
isn't the fault of your patch, but it's a little odd. That being said, I'm
not sure there's a problem with either approach.

+if host_cpu == 'x86' or host_cpu == 'x86_64'
+ pgport_sources += files(
+ 'pg_crc32c_sse42_choose.c',
+ 'pg_crc32c_sse42.c',
+ )
+endi

We could probably just build these unconditionally (i.e., put them in the
first list of pgport_sources in this file) instead of keeping this
complexity in the build scripts.

+#if defined(USE_SSE42_CRC32C) || defined(USE_SSE42_CRC32C_WITH_RUNTIME_CHECK)

Can we move this to just below #include "c.h"? The reason I didn't do this
for the AVX-512 stuff is because TRY_POPCNT_FAST is defined in
pg_bitutils.h, but looking again, I may be able to at least move the check
for USE_AVX512_POPCNT_WITH_RUNTIME_CHECK up similarly.

--
nathan