Make fallback MD5 implementation thread-safe on big-endian systems

Make fallback MD5 implementation thread-safe on big-endian systems

Replace a static scratch buffer with a local variable, because a
static buffer makes the function not thread-safe. This function is
used in client-code in libpq, so it needs to be thread-safe. It was
until commit b67b57a966, which replaced the implementation with the
one from pgcrypto.

Backpatch to v14, where we switched to the new implementation.

Reviewed-by: Robert Haas, Michael Paquier
Discussion: https://www.postgresql.org/message-id/dfa2015d-ad21-4802-a4cc-3850fc5fff3f@iki.fi