Add support to the RX toolchain to restrict the use of string instructions.

Add support to the RX toolchain to restrict the use of string instructions.

bfd	* elf32-rx.c (describe_flags): Report the settings of the string
	insn using bits.
	(rx_elf_merge_private_bfd_data): Handle merging of the string insn
	using bits.

bin	* readelf.c (get_machine_flags): Report the setting of the string
	insn using bits.

gas	* config/tc-rx.c (enum options): Add OPTION_DISALLOW_STRING_INSNS.
	(md_longopts): Add -mno-allow-string-insns.
	(md_parse_option): Handle -mno-allow-string-insns.
	(md_show_usage): Mention -mno-allow-string-insns.
	(rx_note_string_insn_use): New function.  Produces an error
	message if a string insn is used when it is not allowed.
	* config/rx-parse.y (SCMPU): Call rx_note_string_insn_use.
	(SMOVU, SMOVB, SMOVF, SUNTIL, SWHILE, RMPA): Likewise.
	* config/rx-defs.h (rx_note_string_insn_use): Prototype.
	* doc/c-rx.texi: Document -mno-allow-string-insns.

elf	* rx.h (E_FLAG_RX_SINSNS_SET): New bit in e_flags field.
	(E_FLAG_RX_SINSNS_YES): Likewise.
	(E_FLAG_RX_SINSNS_MASK): New define.