Implement the -fuse-ld= option.

This commit implements the -fuse-ld= option, so that the user
can specify -fuse-ld=bfd to use ld.bfd.

This commit re-applies r194328 with some test case changes.
It seems that r194328 was breaking macosx or mingw build
because clang can't find ld.bfd or ld.gold in the given sysroot.
We should use -B to specify the executable search path instead.

Patch originally by David Chisnall.