This is intended to be used for Linux kernel RETHUNK builds. With this commit (optionally backported to Rust 1.73.0), plus a patched Linux kernel to pass the flag, I get a RETHUNK build with Rust enabled that is `objtool`-warning-free and is able to boot in QEMU and load a sample Rust kernel module. Signed-off-by: Miguel Ojeda <ojeda@kernel.org>