Add -Zfunction-return={keep,thunk-extern} option

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>

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -200,6 +200,7 @@ pub enum AttributeKind {
     AllocatedPointer = 38,
     AllocAlign = 39,
     SanitizeSafeStack = 40,
+    FnRetThunkExtern = 41,
 }
 
 /// LLVMIntPredicate