PR24390, Don't decode mtfsb field as a cr field

PR24390, Don't decode mtfsb field as a cr field

"mtfsb0 4*cr7+lt" doesn't make all that much sense, but unfortunately
glibc uses just that instead of "mtfsb0 28" to clear the fpscr xe bit.
So for backwards compatibility accept cr field expressions when
assembling mtfsb operands, but disassemble to a plain number.

	PR 24390
include/
	* opcode/ppc.h (PPC_OPERAND_CR_REG): Comment.
opcodes/
	* ppc-opc.c (BTF): Define.
	(powerpc_opcodes): Use for mtfsb*.
	* ppc-dis.c (print_insn_powerpc): Print fields with both
	PPC_OPERAND_CR_REG and PPC_OPERAND_CR_BIT as a plain number.
gas/
	* testsuite/gas/ppc/476.d: Update mtfsb*.
	* testsuite/gas/ppc/a2.d: Likewise.