x86/retpolines: Enable the default thunk warning only on relevant configs

The using-default-thunk warning check makes sense only with
configurations which actually enable the special return thunks.

Otherwise, it fires on unrelated 32-bit configs on which the special
return thunks won't even work (they're 64-bit only) and, what is more,
those configs even go off into the weeds when booting in the
alternatives patching code, leading to a dead machine.

Fixes: 4461438a8405 ("x86/retpoline: Ensure default return thunk isn't used at runtime")
Reported-by: Klara Modin <[e-mail]>
Reported-by: Erhard Furtner <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Tested-by: Klara Modin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/r/20240413024956.488d474e@yea

arch/x86/lib/retpoline.S

···
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
+#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
+    defined(CONFIG_MITIGATION_SRSO) || \
+    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
 	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
 		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
+#else
+	ANNOTATE_UNRET_SAFE
+	ret
+#endif
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)