x86/rust: support MITIGATION_RETHUNK

The Rust compiler added support for `-Zfunction-return=thunk-extern` [1]
in 1.76.0 [2], i.e. the equivalent of `-mfunction-return=thunk-extern`.
Thus add support for `MITIGATION_RETHUNK`.

Without this, `objtool` would warn if enabled for Rust and already warns
under IBT builds, e.g.:

    samples/rust/rust_print.o: warning: objtool:
    _R...init+0xa5c: 'naked' return found in RETHUNK build

Link: rust-lang/rust#116853 [1]
Link: rust-lang/rust#116892 [2]
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

arch/x86/Makefile

@@ -24,11 +24,15 @@ RETPOLINE_CFLAGS	+= $(call cc-option,-mindirect-branch-cs-prefix)
 
 ifdef CONFIG_MITIGATION_RETHUNK
 RETHUNK_CFLAGS		:= -mfunction-return=thunk-extern
+RETHUNK_RUSTFLAGS	:= -Zfunction-return=thunk-extern
 RETPOLINE_CFLAGS	+= $(RETHUNK_CFLAGS)
+RETPOLINE_RUSTFLAGS	+= $(RETHUNK_RUSTFLAGS)
 endif
 
 export RETHUNK_CFLAGS
+export RETHUNK_RUSTFLAGS
 export RETPOLINE_CFLAGS
+export RETPOLINE_RUSTFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
 # For gcc stack alignment is specified with -mpreferred-stack-boundary,
@@ -218,6 +222,7 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_MITIGATION_RETPOLINE
   KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
+  KBUILD_RUSTFLAGS += $(RETPOLINE_RUSTFLAGS)
   # Additionally, avoid generating expensive indirect jumps which
   # are subject to retpolines for small number of switch cases.
   # LLVM turns off jump table generation by default when under