[patch][gold] Fix PR11604

[Cary Coutant]

> With the attached patch gold can, with the llvm plugin, pass the test in
>
> http://llvm.org/releases/2.6/docs/LinkTimeOptimization.html
>
> The last remaining issue was that the delayed layout would add back a
> section the garbage collector decided was not needed.
>
> I have ported the test to use the test plugin by checking in the original
> .syms file but a .c file that reflects what LTO can do with it.

The fix to Sized_relobj::do_layout_deferred_sections looks good to me (thanks!).

In the test case, plugin_test_7_1.syms should be derived automatically
from plugin_test_7_1.o, rather than checked in as a source file.

-cary