Fix a potential deadlock in some older Loongson 3A1000 MIPS processors.

Fix a potential deadlock in some older Loongson 3A1000 MIPS processors.

	* NEWS: Mention -m[no-]fix-loongson3-llsc.
	* configure.ac: Add --enable-mips-fix-loongson3-llsc.
	Define DEFAULT_MIPS_FIX_LOONGSON3_LLSC.
	* config.in: Regenerated.
	* configure: Likewise.
	* config/tc-mips.c (sync_insn, mips_fix_loongson3_llsc):
	New variables.
	(options): New OPTION_FIX_LOONGSON3_LLSC,
	OPTION_NO_FIX_LOONGSON3_LLSC.
	(md_longopts): Add -m[no-]fix-loongson3-llsc.
	(md_begin): Initialize sync insn.
	(fix_loongson3_llsc): New.
	(append_insn): Call fix_loongson3_llsc.
	(md_parse_option): Handle OPTION_FIX_LOONGSON3_LLSC,
	OPTION_NO_FIX_LOONGSON3_LLSC.
	(md_show_usage): Display -m[no-]fix-loongson3-llsc.
	* doc/c-mips.texi: Document -m[no-]fix-loongson3-llsc,
	--enable-mips-fix-loongson3-llsc=[yes|no].