Allow for optional operands with non-zero default values.

ISA 2.07 (ie, POWER8) added the rfebb instruction which takes one operand
with the value of either a 0 or 1.  It also defines an extended mnemonic
with no operands (ie, "rfebb") that is supposed to be equivalent to "rfebb 1".
I implemented rfebb's lone operand with PPC_OPERAND_OPTIONAL, but the
problem is, optional operands that are ommitted always default to the
value 0, which is wrong in this case.  I have added support for allowing
non-zero default values by adding an additional flag PPC_OPERAND_OPTIONAL_VALUE
that specifies that the default operand value to be used is stored in the
SHIFT field of the operand field immediately following this one.

This fixes the rfebb issue.  I also fixed the mftb and mfcr instructions
so they use the same mechanism.  This allows us to flag invalid uses of
mfcr where we explicitly pass in a zero FXM value, like the use in a2.[sd].

include/opcode/

	* ppc.h (PPC_OPERAND_OPTIONAL_VALUE): New.
	(ppc_optional_operand_value): New inline function.

opcodes/
	* ppc-dis.h (skip_optional_operands): Use ppc_optional_operand_value.
	* ppc-opc.c (FXM4): Add non-zero optional value.
	(TBR): Likewise.
	(SXL): Likewise.
	(insert_fxm): Handle new default operand value.
	(extract_fxm): Likewise.
	(insert_tbr): Likewise.
	(extract_tbr): Likewise.

gas/
	* config/tc-ppc.c (md_assemble): Use ppc_optional_operand_value.
	Allow for optional operands without insert functions.

gas/testsuite/
	* gas/ppc/power8.d: Fixup rfebb test results.
	* gas/ppc/a2.s: Fix invalid mfcr test.
	* gas/ppc/a2.d: Likewise.

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