pgsql: pg_attribute_no_sanitize_alignment() macro

pg_attribute_no_sanitize_alignment() macro

Modern gcc and clang compilers offer alignment sanitizers, which help to detect
pointer misalignment.  However, our codebase already contains x86-specific
crc32 computation code, which uses unalignment access.  Thankfully, those
compilers also support the attribute, which disables alignment sanitizers at
the function level.  This commit adds pg_attribute_no_sanitize_alignment(),
which wraps this attribute, and applies it to pg_comp_crc32c_sse42() function.

Back-patch of commits 993bdb9f9 and ad2ad698a, to enable doing
alignment testing in all supported branches.

Discussion: https://postgr.es/m/CAPpHfdsne3%3DT%3DfMNU45PtxdhSL_J2PjLTeS8rwKnJzUR4YNd4w%40mail.gmail.com
Discussion: https://postgr.es/m/475514.1612745257%40sss.pgh.pa.us
Author: Alexander Korotkov, revised by Tom Lane
Reviewed-by: Tom Lane

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cc7ea0717b128cb23d564b9b7c899eb6164752fc

Modified Files
--------------
src/include/c.h            | 12 ++++++++++++
src/port/pg_crc32c_sse42.c |  1 +
2 files changed, 13 insertions(+)