[CMake] LLVMSupport: Give system_libs PRIVATE scope when LLVMSupport is built as SHARED. Users of LLVMSupport won't inherit ${system_libs}.

unittests/SupporTests is another user of libpthreads. Apply LLVM_SYSTEM_LIBS for him explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221531 91177308-0d34-0410-b5e6-96231b3b80d8