[PATCH][MIPS] GAS: Fix Loongson3 LLSC errata.

[Paul Hua]

Hi:

In some older Loongson 3A1000 processors there is a LL/SC errata that
can cause the CPU to deadlock occasionally.  The details are very
complicated. We find a way to work around this errata by adding a sync
before ll/lld instruction.

This patch also add a configure options
--enable-mips-fix-loongson3-llsc=[yes|no] to enable fix-loongson3-llsc
by config.

[MIPS] GAS: Fix Loongson3 LLSC errata.

gas/
        * 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].
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