Thread-unsafe MD5 on big-endian systems with no OpenSSL

While browsing through all our global variables for the multithreading
effort, I noticed that our MD5 implementation in src/common/md5.c uses a
static buffer on big-endian systems, which makes it not thread-safe.
That's a bug because that function is also used in libpq.

This was introduced in commit b67b57a966, which replaced the old MD5
fallback implementation with the one from pgcrypto. The thread-safety
didn't matter for pgcrypto, but for libpq it does.

This only affects big-endian systems that are compiled without OpenSSL.

--
Heikki Linnakangas
Neon (https://neon.tech)