[PowerPC] Add support for high and higha symbol modifiers on tls modifers.

Enables using the high and high-adjusted symbol modifiers on thread local
storage modifers in powerpc assembly. Needed to be able to support 64 bit
thread-pointer and dynamic-thread-pointer access sequences.

Differential Revision: https://reviews.llvm.org/D47754

llvm-svn: 334856