depends: Decouple toolchain + binutils

For now they remain the same, but in the next commit, we will assign
them differently according to wether or not we're using system clang.

depends/hosts/darwin.mk

@@ -20,5 +20,6 @@ darwin_release_CXXFLAGS=$(darwin_release_CFLAGS)
 darwin_debug_CFLAGS=-O1
 darwin_debug_CXXFLAGS=$(darwin_debug_CFLAGS)
 
+darwin_native_binutils=native_cctools
 darwin_native_toolchain=native_cctools
 darwin_cmake_system=Darwin