cmake: remove llvm runtime library selection from target_base.cmake

The selection of the runtime library when using LLVM has already been
done in llvm/target.cmake and passed to the toolchain_ld_base() function
through the global TOOLCHAIN_LD_FLAGS setting.

The only reason this haven't been noticed is because of CMake's built-in
symbol de-duplication feature.

Remove the redundant runtime library handling.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>