[MIPS/GAS] Add Loongson EXT2 Instructions support.

bfd/
	* elfxx-mips.c (print_mips_ases): Add Loongson EXT2 extension.

binutils/
	* readelf.c (print_mips_ases): Add Loongson EXT2 extension.

gas/
	* NEWS: Mention Loongson EXTensions R2 (EXT2) support.
	* config/tc-mips.c (options): Add OPTION_LOONGSON_EXT2 and
	OPTION_NO_LOONGSON_EXT2.
	(md_longopts): Likewise.
	(mips_ases): Define availability for EXT.
	(mips_convert_ase_flags): Map ASE_LOONGSON_EXT2 to
	AFL_ASE_LOONGSON_EXT2.
	(md_show_usage): Add help for -mloongson-ext2 and
	-mno-loongson-ext2.
	* doc/as.texi: Document -mloongson-ext2, -mno-loongson-ext2.
	* doc/c-mips.texi: Document -mloongson-ext2, -mno-loongson-ext2,
	.set loongson-ext2 and .set noloongson-ext2.
	* testsuite/gas/mips/loongson-ext2.d: New test.
	* testsuite/gas/mips/loongson-ext2.s: New test.
	* testsuite/gas/mips/mips.exp: Run loongson-ext2 test.

include/
	* elf/mips.h (AFL_ASE_LOONGSON_EXT2): New macro.
	(AFL_ASE_MASK): Update to include AFL_ASE_LOONGSON_EXT2.
	* opcode/mips.h (ASE_LOONGSON_EXT2): New macro.

opcodes/
	* mips-dis.c (parse_mips_ase_option): Handle -M loongson-ext
	option.
	(print_mips_disassembler_options): Document -M loongson-ext.
	* mips-opc.c (LEXT2): New macro.
	(mips_opcodes): Add cto, ctz, dcto, dctz instructions.