[compiler-rt] Build libfuzzer sources with the chosen C++ compiler

I was getting build failures due to missing <cstddef> when building the
libfuzzer tests. It turns out that the custom command was using
COMPILER_RT_TEST_COMPILER when building the source file rather than
the COMPILER_RT_TEST_CXX_COMPILER.

Pull Request: https://github.com/llvm/llvm-project/pull/83090