Update ARCH_FLAGs and some old 64-bit related comments.

An external contributor does some work to revive the x86_64 port. Interpreter is only a primary stage. Situation is the same as 5 years ago, nothings work on 64 bits from an user point of view. Once recompilers are ported to 64bits, I might agree to change the warning. It is too soon for the moment.