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

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.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D120946

Author: colincross

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -602,6 +602,9 @@ macro(add_custom_libcxx name prefix)
     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