[gold][patch] Add a plugin callback for setting the extra search path

[Cary Coutant]

> The attached patch includes the changes you asked for. The main reason
> why I moved find_file out of open was to use early return statements
> when we find the file.

OK.

+static bool
+find_file(const Dirsearch& dirpath, int *pindex,
+          const Input_file_argument* input_argument, bool *is_in_sysroot,
+          std::string *found_name, std::string *namep)

Space goes after the "*" for C++ code in gold (4x).

It looks good to me with that change.

-cary