[PowerPC] Add some existing instructions to ppc64-encoding-bookII.s

The test case had a couple of FIXMEs where the instruction is in
fact already supported by the back-end.  In some other case, while
the generic form of the instruction is not yet supported, a
specialized form is.  This adds tests for those already supported
instructions / instruction forms.
Added c++ mode selector to head of SelectionDAGBuilder.h so editors open it in c++ mode instead of c mode.