Add an extra-library-path option to the plugin. This is used to support

having a library both as bitcode and native code. We want to use the
bitcode first, but if codegen produces new undefined references we have to use
the native code to satisfy those references.

Gold has no notion of bitcode and native search directories, so instead it has
an API where the plugin can instruct it to look for the libraries it is passing
to it. This patch uses that API.

llvm-svn: 106674