PowerPC: Add support for RFC02655 - Saturating Subtract Instruction

opcodes/
	* ppc-opc.c (XOL): New define.
	(XOL_MASK): Likewise.
	(powerpc_opcodes): Add subfus, subfus., subwus, subwus., subdus, subdus.

gas/
	* testsuite/gas/ppc/rfc02655.s: New test.
	* testsuite/gas/ppc/rfc02655.d: Likewise
	* testsuite/gas/ppc/future-raw.s: Likewise.
	* testsuite/gas/ppc/future-raw.d: Likewise.
	* testsuite/gas/ppc/ppc.exp: Run them.