databases/db5, db6: enable DBM compatibility

Enable DBM API. [1]
Reorder license information of db5/Makefile to fix portlint warning. [1]
Bump PORTREVISON. [1]
Add command creating symbolic link such as ${PREFIX}/lib/libdb-6.so to
post-install target of db6/Makefile. [1]

Instead of USE_GCC, patch src/dbinc/db.in to not expose the store() macro under C++.
Mark db6 broken on MIPS64 for now, we do not have a workable MUTEX implementation.
Unbreak build on AARCH64, where the db6 autoconfiguration chooses the wrong MUTEX.

PR: 230191
Submitted by: Yasuhiro Kimura