If lld binary is named 'ld' on darwin, use darwin driver mode

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217112 91177308-0d34-0410-b5e6-96231b3b80d8
Fix unit test now that 'ld' is host specific

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217114 91177308-0d34-0410-b5e6-96231b3b80d8