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

LLVM libs are printed in the first line, and system libs are printed in the next line.

  $ bin/llvm-config --libs object
  -lLLVMObject -lLLVMSupport
  -lrt -ldl -ltinfo -lpthread -lz

It is workaround for PR3347 and PR8449.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197380 91177308-0d34-0410-b5e6-96231b3b80d8