pgsql: Silence warning in older versions of Valgrind

Silence warning in older versions of Valgrind

Due to misunderstanding on my part, commit 235328ee4 did not go far
enough to silence older versions of Valgrind. For those, it was the bit
scan that was problematic, not the subsequent bit-masking operation. To
fix, use the unaligned path for the trailing bytes. Since we don't have
a bit scan here anymore, also remove some comments and endian-specific
coding around that.

Reported-by: Anton A. Melnikov <a(dot)melnikov(at)postgrespro(dot)ru>
Discussion: https://postgr.es/m/f3aa2d45-3b28-41c5-9499-a1bc30e0f8ec@postgrespro.ru
Backpatch-through: 17

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fde7c0164ea20433964a0a05a91c5708384a285c

Modified Files
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src/include/common/hashfn_unstable.h | 26 +++-----------------------
1 file changed, 3 insertions(+), 23 deletions(-)