s390: Add arch15 Concurrent-Functions Facility insns

opcodes/
	* s390-opc.txt: Add arch15 Concurrent-Functions Facility
	instructions.
	* s390-opc.c (INSTR_SSF_RRDRD2, MASK_SSF_RRDRD2): New SSF
	instruction format variant.

gas/testsuite/
	* gas/s390/zarch-arch15.d: Tests for arch15 Concurrent-Functions
	Facility instructions.
	* gas/s390/zarch-arch15.s: Likewise.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>