[gold][patch] Add a add_input_library callback

[Cary Coutant]

>> Please make sure that http://gcc.gnu.org/wiki/whopr/driver is up to
>> date with this and other changes, or get Cary to do it.  In
>> particular, LDPT_ADD_INPUT_LIBRARY needs to be documented to explain
>> what sort of file name should be passed--e.g., "c" to mean the same as
>> "-lc".
>
> I have updated the wiki. Let me know if you think I should change something.

Where it says: "pathname: The name of the library without the lib
prefix", I think it should also say "and without the extension", or
you could say something like "the name of the library as passed to the
linker in the -l option." Given that, "pathname" probably isn't the
right choice here for the parameter name.

-cary