[arm] Refine ARMBuildAttrs.h.

This commit cleans up some comments in ARMBuildAttrs.h.
Besides, this commit fixes an error related to AllowWMMXv1
and AllowWMMXv2 (although they are not used currently.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194327 91177308-0d34-0410-b5e6-96231b3b80d8