[PPC64] Support "symbol@high" and "symbol@higha" symbol modifers.

Add support for the "@high" and "@higha" symbol modifiers in powerpc64 assembly.
The modifiers represent accessing the segment consiting of bits 16-31 of a
64-bit address/offset.

Differential Revision: https://reviews.llvm.org/D47729

llvm-svn: 334855