Add option -mfence-as-lock-add=[no|yes].

Add option -mfence-as-lock-add=[no|yes].

With -mfence-as-lock-add=yes lfence, mfence and sfence will be encoded
as lock addl $0x0, (%{r,e}sp).

gas/:

    * config/tc-i386.c (avoid_fence): New.
    (output_insn): Encode as lock addl $0x0, (%{r,e}sp) if avoid_fence
    is true.
    (OPTION_FENCE_AS_LOCK_ADD): New.
    (md_longopts): Add -mfence-as-lock-add.
    (md_parse_option): Handle -mfence-as-lock-add.
    (md_show_usage): Add -mfence-as-lock-add=[no|yes].
    * doc/c-i386.texi (-mfence-as-lock-add): Document.

gas/testsuite/:

    * gas/i386/i386.exp: Run new tests.
    * gas/i386/fence-as-lock-add.s: New.
    * gas/i386/fence-as-lock-add-yes.d: Likewise.
    * gas/i386/fence-as-lock-add-no.d: Likewise.
    * gas/i386/x86-64-fence-as-lock-add-yes.d: Likewise.
    * gas/i386/x86-64-fence-as-lock-add-no.d: Likewise.