ppc: Document the -mfuture and -Mfuture options and make them usable

ppc: Document the -mfuture and -Mfuture options and make them usable

The -mfuture and -Mfuture options which are used for adding potential
new ISA instructions were not documented.  They also lacked a bitmask
so new instructions could not be enabled by those options.  Fixed.

binutils/
	* doc/binutils.texi: Document -Mfuture.

gas/
	* config/tc-ppc.c: Document -mfuture
	* doc/c-ppc.texi: Likewise.

include/
	* opcode/ppc.h (PPC_OPCODE_FUTURE): Define.

opcodes/
	* ppc-dis.c (ppc_opts) <future>: Use it.
	* ppc-opc.c (FUTURE): Define.