[PowerPC] Add some Book II instructions to AsmParser

This patch adds a couple of Book II instructions (isync, icbi) to the
PowerPC assembler parser.  These are needed when bootstrapping clang
with the integrated assembler forced on, because they are used in
inline asm statements in the code base.

The test case adds the full list of Book II storage control instructions,
including associated extended mnemonics.  Again, those that are not yet
supported as marked as FIXME.