[PowerPC] Support @toc@h modifier

This adds the relocation type and other necessary infrastructure
to use the @toc@h modifier in the assembler.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184549 91177308-0d34-0410-b5e6-96231b3b80d8

[PowerPC] Support @higher et.al. modifiers

This adds support for the @higher, @highera, @highest, and @highesta
modifers, including some missing relocation types.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184550 91177308-0d34-0410-b5e6-96231b3b80d8