[PowerPC][MC] Set 'future' as default CPU type of XCOFF

For Power on Linux (both LE and BE), ELFObjectFile returns 'future' as
default CPU type if mcpu is not specified, so that all necessary
features will be enabled in MC.

While for XCOFF, the default CPU type is always null, which makes tools
like llvm-objdump not able to recognize prefixed instructions, unless
specifying --mcpu=pwr10 or --mattr=+prefix-instrs manually.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D155089