POWER10: Add Return-Oriented Programming instructions

POWER10 adds some return-oriented programming (ROP) instructions and
this patch adds support for them.  You will notice that they are enabled
for POWER8 and later, not just POWER10 and later.  This is on purpose.
This allows the instructions to be added to POWER8 binaries that can be
run on POWER8, POWER9 and POWER10 cpus.  On POWER8 and POWER9, these
instructions just act as nop's.

opcodes/
	* ppc-opc.c (insert_dw, (extract_dw): New functions.
	(DW, (XRC_MASK): Define.
	(powerpc_opcodes) <hashchk, hashchkp, hashst, haststp>: New mnemonics.
gas/
	* testsuite/gas/ppc/rop-checks.d,
	* testsuite/gas/ppc/rop-checks.l,
	* testsuite/gas/ppc/rop-checks.s,
	* testsuite/gas/ppc/rop.d,
	* testsuite/gas/ppc/rop.s: New tests.
	* testsuite/gas/ppc/ppc.exp: Run them.