xtensa constify

Move lots of read-only arrays to .rodata.

include/
	* xtensa-isa-internal.h (xtensa_format_internal),
	(xtensa_slot_internal, xtensa_operand_internal),
	(xtensa_arg_internal, xtensa_iclass_internal),
	(xtensa_opcode_internal, xtensa_regfile_internal),
	(xtensa_interface_internal, xtensa_funcUnit_internal),
	(xtensa_state_internal, xtensa_sysreg_internal): Constify.
bfd/
	* elf32-xtensa.c (narrowable, widenable): Constify.
	* xtensa-modules.c: Constify many arrays.