[MIPS/GAS] Split Loongson EXT Instructions from loongson3a.

bfd/
	 * elfxx-mips.c (infer_mips_abiflags): Use ases instead of
	 isa_ext for infer ABI flags.
	 (print_mips_ases): Add Loongson EXT extension.

binutils/
	 * readelf.c (print_mips_ases): Add Loongson EXT extension.

elfcpp/
	 * mips.h (AFL_ASE_LOONGSON_EXT): New enum.

gas/
	 * NEWS: Mention Loongson EXTensions (EXT) support.
	 * config/tc-mips.c (options): Add OPTION_LOONGSON_EXT and
	 OPTION_NO_LOONGSON_EXT.
	 (md_longopts): Likewise.
	 (mips_ases): Define availability for EXT.
	 (mips_convert_ase_flags): Map ASE_LOONGSON_EXT to
	 AFL_ASE_LOONGSON_EXT.
	 (mips_cpu_info_table): Add ASE_LOONGSON_EXT for loongson3a.
	 (md_show_usage): Add help for -mloongson-ext and
	 -mno-loongson-ext.
	 * doc/as.texi: Document -mloongson-ext, -mno-loongson-ext.
	 * doc/c-mips.texi: Document -mloongson-ext, -mno-loongson-ext,
	 .set loongson-ext and .set noloongson-ext.
	 * testsuite/gas/mips/loongson-mmi.d: Add ASE flag.

include/
	 * elf/mips.h (AFL_ASE_LOONGSON_EXT): New macro.
	 (AFL_ASE_MASK): Update to include AFL_ASE_LOONGSON_EXT.
	 * opcode/mips.h (ASE_LOONGSON_EXT): New macro.

opcodes/
	 * mips-dis.c (mips_arch_choices): Add EXT to loongson3a
	 descriptors.
	 (parse_mips_ase_option): Handle -M loongson-ext option.
	 (print_mips_disassembler_options): Document -M loongson-ext.
	 * mips-opc.c (IL3A): Delete.
	 * mips-opc.c (LEXT): New macro.
	 (mips_opcodes): Replace IL2F|IL3A marking with LEXT for EXT
	 instructions.