pgsql: Ensure USE_AVX... symbols are not defined if not building for x8

Ensure USE_AVX... symbols are not defined if not building for x86_64.

Various code assumed this was true already, and usually it is.
However, it emerges that in a "universal" (multi-architecture)
macOS build, configure will define USE_AVX2_WITH_RUNTIME_CHECK
if the build host is x86_64, and then the arm64 half of the
build fails.

Ideally we'd get pg_config.h to define this symbol conditionally
depending on defined(__x86_64__), but I don't see any way to
persuade Autoconf to do that. Instead, clean up the mess by
#undef'ing it again in c.h for not-x86_64 builds.

For consistency I made c.h also #undef the USE_AVX512... symbols.
Those are not actively broken, but it seems only happenstance
that configure's tests for them fail in a universal build.
Down the road we may have occasion to add more #undef's here.

This problem is new in v19, so no need for back-patch.

Reported-by: Sandeep Thakkar <sandeep(dot)thakkar(at)enterprisedb(dot)com>
Reported-by: Tobias Bussmann <t(dot)bussmann(at)gmx(dot)net>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/15574903-87C9-478A-B2D7-CC8F4C275DBB@gmx.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e88bd2736f86d66dc75ad8b2f7edd799d20b7d53

Modified Files
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src/include/c.h | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)