Darwin: use the compiler-provided linker

When actually compiling we use clang for the final .dylib creation stage, so we
should ask it what linker it'll use when detecting support for our
architectures.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201845 91177308-0d34-0410-b5e6-96231b3b80d8