llvm-config: Introduce --system-libs to print SYSTEM_LIBS, and deprecate SYSTEM_LIBS in --ldflags.

Although --system-libs is expected to use after --libs, it can be used alone.

  $ bin/llvm-config --ldflags
  -L/path/to/llvm/objroot/lib

  $ bin/llvm-config --libs object
  -lLLVMObject -lLLVMSupport

  $ bin/llvm-config --system-libs
    (Blank line. "all" is assumed but nothing is printed.)
  -lrt -ldl -ltinfo -lpthread -lz

  $ bin/llvm-config --ldflags --libs --system-libs object
  -L/path/to/llvm/objroot/lib
  -lLLVMObject -lLLVMSupport
  -lrt -ldl -ltinfo -lpthread -lz

It is reimplementation of r197380, and workaround for PR3347 and PR8449.

FIXME: Each LLVM component may have its dependent system libs.
llvm-svn: 197664