pgsql: Add optimized functions for linear search within byte arrays

Add optimized functions for linear search within byte arrays

In similar vein to b6ef167564, add pg_lfind8() and pg_lfind8_le()
to search for bytes equal or less-than-or-equal to a given byte,
respectively. To abstract away platform details, add helper functions
and typedefs to simd.h.

John Naylor and Nathan Bossart, per suggestion from Andres Freund

Discussion:
https://www.postgresql.org/message-id/CAFBsxsGzaaGLF%3DNuq61iRXTyspbO9rOjhSqFN%3DV6ozzmta5mXg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e813e0e16852c080259cd0813e1a82ecb2625aea

Modified Files
--------------
src/include/port/pg_lfind.h                        |  68 ++++++++-
src/include/port/simd.h                            | 168 ++++++++++++++++++++-
.../modules/test_lfind/expected/test_lfind.out     |  18 ++-
src/test/modules/test_lfind/sql/test_lfind.sql     |   4 +-
src/test/modules/test_lfind/test_lfind--1.0.sql    |  10 +-
src/test/modules/test_lfind/test_lfind.c           | 100 +++++++++++-
6 files changed, 358 insertions(+), 10 deletions(-)