[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334855 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed