Provide a hook to customize missing library error handling

Make it possible for lld users to provide a custom script that would help to
find missing libraries. A possible scenario could be:

    % clang /tmp/a.c -fuse-ld=lld -loauth -Wl,--error-handling-script=/tmp/addLibrary.py
    unable to find library -loauth
    looking for relevant packages to provides that library

        liboauth-0.9.7-4.el7.i686
        liboauth-devel-0.9.7-4.el7.i686
        liboauth-0.9.7-4.el7.x86_64
        liboauth-devel-0.9.7-4.el7.x86_64
        pix-1.6.1-3.el7.x86_64

Where addLibrary would be called with the missing library name as first argument
(in that case addLibrary.py oauth)

Differential Revision: https://reviews.llvm.org/D87758