[MC] Support @ variants with directional labels

The assembler parser common code supports recognizing symbol variants
using the @ modifer.  On PowerPC, it should also be possible to use
(some of) those modifiers with directional labels, like "1f@l".

This patch adds support for accepting symbol variants on directional
labels as well.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184437 91177308-0d34-0410-b5e6-96231b3b80d8