For Darwin builds, locate tools using xcrun when available.

The make/platform/darwin_bni.mk file already has similar code but we
apparently neglected to add it to the clang_darwin.mk file.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@188864 91177308-0d34-0410-b5e6-96231b3b80d8