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

[ccross]

Summary

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.

[phosek]

LGTM

compiler-rt/cmake/Modules/AddCompilerRT.cmake

set(PASSTHROUGH_VARIABLES
     CMAKE_RANLIB
     CMAKE_NM
     CMAKE_OBJCOPY
     CMAKE_OBJDUMP
     CMAKE_STRIP
     CMAKE_READELF
     CMAKE_SYSROOT
     LIBCXX_HAS_MUSL_LIBC
+    LIBCXX_HAS_GCC_S_LIB
     LIBCXX_HAS_PTHREAD_LIB
     LIBCXX_HAS_RT_LIB
+    LIBCXX_USE_COMPILER_RT
     LIBCXXABI_HAS_PTHREAD_LIB
     PYTHON_EXECUTABLE
     Python3_EXECUTABLE
     Python2_EXECUTABLE
     CMAKE_SYSTEM_NAME)
   foreach(variable ${PASSTHROUGH_VARIABLES})
     get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)
     if(${is_value_set})