[lldb] Use system c++ lib for LLDB STL tests

This change partially reverts #112357 to avoid LLDB STL test
failures on machines which do not have gcc installed.

Author: zeroomega

lldb/test/API/lang/cpp/stl/Makefile

@@ -1,9 +1,3 @@
 CXX_SOURCES := main.cpp
 
-ifneq ($(OS),Darwin)
-    USE_LIBSTDCPP := 1
-else
-    USE_SYSTEM_STDLIB := 1
-endif
-
 include Makefile.rules