ARM: mark various aliases with their architecture requirements.

If an alias inherits directly from InstAlias then it doesn't get any default
"Requires" values, so llvm-mc will allow it even on architectures that don't
support the underlying instruction.

This tidies up the obvious VFP and NEON cases I found.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193340 91177308-0d34-0410-b5e6-96231b3b80d8