[PPC64] Fix DQ-form instruction handling and emit error for misalignment.

Relanding r340564, original commit message:

Fixes the handling of *_DS relocations used on DQ-form instructions where we
were overwriting some of the extended opcode bits. Also adds an alignment check
so that the user will receive a diagnostic error if the value we are writing
is not properly aligned.

Differential Revision: https://reviews.llvm.org/D51124

llvm-svn: 340832