[PowerPC] Support ".machine any"

The PowerPC assembler is supposed to provide a directive .machine
that allows switching the supported CPU instruction set on the fly.
Since we do not yet check CPU feature sets at all and always accept
any available instruction, this is not really useful at this point.

However, it makes sense to accept (and ignore) ".machine any" to
avoid spuriously rejecting existing assembler files that use this.

llvm-svn: 185924