TableGen: allow use of uint64_t for available features mask.

ARM in particular is getting dangerously close to exceeding 32 bits worth of
possible subtarget features. When this happens, various parts of MC start to
fail inexplicably as masks get truncated to "unsigned".

Mostly just refactoring at present, and there's probably no way to test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215887 91177308-0d34-0410-b5e6-96231b3b80d8