Generate C++ thunks for Rust protos

In this CL I'd like to call existing C++ Protobuf API from the V0 Rust API. Since parts of the C++ API are defined inline and using (obviously) C++ name mangling, we need to create a "thunks.cc" file that:

1) Generates code for C++ API function we use from Rust
2) Exposes these functions without any name mangling (meaning using `extern "C"`)

In this CL we add Bazel logic to generate "thunks" file, compile it, and propagate its object to linking. We also add logic to protoc to generate this "thunks" file.

The protoc logic is rather rudimentary still. I hope to focus on protoc code quality in my followup work on V0 Rust API using C++ kernel.

PiperOrigin-RevId: 523479839

rust/cpp_kernel/rust_alloc_for_cpp_api.rs

@@ -0,0 +1,37 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2023 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+use std::alloc::{alloc, Layout};
+
+#[no_mangle]
+extern "C" fn __pb_rust_alloc(size: usize, align: usize) -> *mut u8 {
+    let layout = Layout::from_size_align(size, align).unwrap();
+    unsafe { alloc(layout) }
+}