ppc: extend opindex to 16 bits

ppc: extend opindex to 16 bits

With the upcoming SVP64 extension[0] to PowerPC architecture, it became
evident that PowerPC operand indices no longer fit 8 bits. This patch
switches the underlying type to uint16_t, also introducing a special
typedef so that any future extension goes even smoother.

[0] https://libre-soc.org

include/
	* opcode/ppc.h (ppc_opindex_t): New typedef.
	(struct powerpc_opcode): Use it.
	(PPC_OPINDEX_MAX): Define.
gas/
	* write.h (struct fix): Increase size of fx_pcrel_adjust.
	Reorganise.
	* config/tc-ppc.c (insn_validate): Use ppc_opindex_t for operands.
	(md_assemble): Likewise.
	(md_apply_fix): Likewise.  Mask fx_pcrel_adjust with PPC_OPINDEX_MAX.
	(ppc_setup_opcodes): Adjust opcode index assertion.
opcodes/
	* ppc-dis.c (skip_optional_operands): Use ppc_opindex_t for
	operand pointer.
	(lookup_powerpc, lookup_prefix, lookup_vle, lookup_spe2): Likewise.
	(print_insn_powerpc): Likewise.