Implement the -fuse-ld= option.

Implement the -fuse-ld= option.

Merge upstream Clang revision 211785:
  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.

This is a direct commit to stable/10 as this is change is already
included in Clang 3.5 in HEAD. The patch is also reworked slightly for
Clang 3.4.1.

Reviewed by:	dim
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@279302 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f