IBM Z: Add risbgz and risbgnz extended mnemonics

IBM Z: Add risbgz and risbgnz extended mnemonics

These two extended mnemonics are documented in the Principles of
Operations manual but currently not supported by Binutils. They
provide aliases for already supported instructions with the zero flag
being set.  The flag otherwise is mingled into one of the immediate
operands what makes asm code much harder to read.

opcodes/

	* s390-opc.txt: Add risbgz and risbgnz.
	* s390-opc.c (U6_26): New operand type.
	(INSTR_RIE_RRUUU2, MASK_RIE_RRUUU2): New instruction format and
	mask.

gas/

	* testsuite/gas/s390/zarch-z10.s: Add tests for risbgz.
	* testsuite/gas/s390/zarch-z10.d: Add regexp for risbgz.
	* testsuite/gas/s390/zarch-zEC12.s: Add tests for risbgnz.
	* testsuite/gas/s390/zarch-zEC12.d: Add regexp for risbgnz.