Revert r197380, "llvm-config: Print SYSTEM_LIBS with --libs, instead of --ldflags."

I will introduce another flag, like --system-libs, later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197583 91177308-0d34-0410-b5e6-96231b3b80d8