Driver: adjust linker invocation for GNUTools

Adjust the driver to invoke the linker more similar to gcc.  -dynamic-linker is
only passed if -static and -shared are not part of the compiler (driver)
invocation.  Replicate the passing of -export-rdynamic as per the GCC link spec:

  %{!static: %{rdynamic:-export-dynamic} %{!shared:-dynamic-linker ...}}

This behaviour is consistent across all the targets that are supported, so no
need to conditionalise it on the target.

Resolves PR24245.

llvm-svn: 260019