Check that the LLVM runtimes checkout is new enough

The code with LLVM 14 (current CHERI master branch) is not new enough to
support testing using the runtimes build, so add a warning and suggest
updating to a newer version (dev branch)

pycheribuild/projects/cross/libcxx.py

@@ -600,6 +600,8 @@ def configure(self, **kwargs) -> None:
                 self.add_cmake_options(LIBCXX_ENABLE_ASSERTIONS=True)
                 # Need to export the symbols from debug.cpp to allow linking code that defines _LIBCPP_DEBUG=1
                 self.add_cmake_options(LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=True)
+        if not GitRepository.contains_commit(self, "f4fdc4f4d9f6903808541645d383be2ee759f400", src_dir=self.source_dir):
+            self.fatal(f"LLVM revision in {self.source_dir} is too old for this target, please update.")
         super().configure(**kwargs)
 
     def compile(self, **kwargs):