Add timingsafe_bcmp(), for constant-time memory comparison

Add timingsafe_bcmp(), for constant-time memory comparison

timingsafe_bcmp() should be used instead of memcmp() or a naive
for-loop, when comparing passwords or secret tokens, to avoid leaking
information about the secret token by timing. This commit just
introduces the function but does not change any existing code to use
it yet.

This has been initially applied as of 09be39112654 in v18 and newer
versions, and will be used in all the stable branches for an upcoming
fix.

Co-authored-by: Jelte Fennema-Nio <github-tech@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/7b86da3b-9356-4e50-aa1b-56570825e234@iki.fi
Security: CVE-2026-6478
Backpatch-through: 14