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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197382
91177308-0d34-0410-b5e6-
96231b3b80d8