Pass through even more LIBCXX_* variables to libfuzzer's custom lib++

Similar to D120946, pass LIBCXX_HAS_GCC_S_LIB and LIBCXX_USE_COMPILER_RT
through to the custom lib++ builds so that libfuzzer doesn't end up with
a .deplibs section that links against those libraries when the
variables are set to false.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D127912