[PowerPC] Add the 'attn' instruction

The attn instruction is not part of the Power ISA, but is documented in the A2
user manual, and is accepted by the GNU assembler for the A2 and the POWER4+.
Reported as part of PR21650.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222712 91177308-0d34-0410-b5e6-96231b3b80d8