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

[ccross]

Summary

Pass LIBCXX_HAS_PTHREAD_LIB, LIBCXX_HAS_RT_LIB and LIBCXXABI_HAS_PTHREAD_LIB
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]

Can you reupload the change with larger context? See https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface.

[phosek]

Do you have commit permissions or do you need me to land this change for you?

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_PTHREAD_LIB
+    LIBCXX_HAS_RT_LIB
+    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})
       get_property(value CACHE ${variable} PROPERTY VALUE)