-Wwrite-strings: Constify struct disassemble_info's disassembler_options field

-Wwrite-strings: Constify struct disassemble_info's disassembler_options field

The memory disassemble_info::disassembler_options points to is always
owned by the client.  I.e., that field is an non-owning, observing
pointer.  Thus const makes sense.

Are the include/ and opcodes/ bits OK?

Tested on x86_64 Fedora 23, built with --enable-targets=all.

include/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* dis-asm.h (disassemble_info) <disassembler_options>: Now a
	"const char *".
	(next_disassembler_option): Constify.

opcodes/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* arc-dis.c (parse_option, parse_disassembler_options): Constify.
	* arm-dis.c (parse_arm_disassembler_options): Constify.
	* ppc-dis.c (powerpc_init_dialect): Constify local.
	* vax-dis.c (parse_disassembler_options): Constify.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* arm-tdep.c (show_disassembly_style_sfunc): Constify local.
	* disasm.c (set_disassembler_options): Constify local.
	* i386-tdep.c (i386_print_insn): Remove cast and FIXME comment.