New gold plugin interface for getting --wrap symbols

[Cary Coutant]

> 2017-11-21  Sriraman Tallam  <tmsriram@google.com>
>
>       * plugin.cc (get_wrap_symbols): New plugin interface.
>       (load): Add get_wrap_symbols to transfer vector.
>       * plugin-api.h (ld_plugin_get_wrap_symbols): New plugin interface.
>       * options.h (options::String_set::size_type): New macro function.

Change this to: (DEFINE_string): Add varname_size() method.

>       * testsuite/plugin_test.c (onload): Call and check get_wrap_symbols
>       interface.
>       * testsuite/plugin_test_wrap_symbols.sh: New test script.
>       * testsuite/plugin_test_wrap_symbols_1.cc: New file.
>       * testsuite/plugin_test_wrap_symbols_2.cc: New file.
>       * testsuite/Makefile.am (plugin_test_wrap_symbols): New test.
>       * testsuite/Makefile.in: Regenerate.

plugin-api.h is in include/ which has its own ChangeLog. That patch
should be applied first and synced with gcc (not sure if they'll take
this during Stage 3, but I think it's harmless enough to be OK). The
gold part is OK after that's checked in.

-cary