Gold Linker Patch: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715 and in some places called "spectre".

[Cary Coutant]

>>> * options.h (retpolineplt): New -z option to use retpoline PLT.
>>> * x86_64.cc (Output_data_plt_x86_64_retpoline): New class.
>>> (Target_x86_64<64>::do_make_data_plt): Create retpoline PLT when
>>> the option is used.
>>> * testsuite/Makefile.am (retpoline_plt_1.sh): New test.
>>> * testsuite/Makefile.in: Regenerate.
>>> * testsuite/retpoline_plt_1.sh: New test script.
>>> * testsuite/retpoline_plt_1.s: New test source.
>>
>> This makes the -z bndplt and -z retpolineplt options mutually
>> exclusive. Please add a check in options.cc
>> (General_options::finalize) for this.
>
> Done and new patch attached.

+  if (this->bndplt() && this->retpolineplt())
+    {
+      gold_fatal(_("-z,bndplt and -z,retpolineplt are mutually exclusive"));
+    }

Replace the commas with spaces -- the options are only spelled with
commas when using the compiler -Wl,... syntax.

OK with that change.

-cary