[PowerPC] Support @h modifier

[PowerPC] Support @h modifier

This adds necessary infrastructure to support the @h modifier.
Note that all required relocation types were already present
(and unused).

This patch provides support for using @h in the assembler;
it would also be possible to now use this feature in code
generated by the compiler, but this is not done yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184548 91177308-0d34-0410-b5e6-96231b3b80d8