llvm-config: Fixup r197380, tweak for cross compilation. SYSTEM_LIBS should be --host's in BuildTools/llvm-config.

FIXME: Host's llvm-config is not generated. It's for target's.
Host tools, aka "BuildTools", in utils, do not require llvm-config to build.

For example with --host=i686-pc-mingw32 --build=linux,

  $ BuildTools/Release+Asserts/bin/llvm-config --libs support
  -lLLVMSupport
  -lpthread -lshell32 -lpsapi -limagehlp -lm