Keep support for the ENABLE64BIT flag (as a flag for choosing between
32 and 64 bit, if the current arch happens to be x86) but avoid using
it in code that wants to know the exact architecture used.
This makes the platform build files more straightforward and
consistent.
This makes sure the code is at least somewhat tested. Ideally those
assembly functions should be upgraded to work in 64 bit mode as well,
but until then it's better to actually use them in the configurations
where they can be built.
This makes it clearer which ones actually are real platforms
(chosen automatically or explicitly via "make ARCH=foo") and which
ones just are shared helpers.
