rust_for_linux: -Zregparm=<N> commandline flag for X86 (#116972)

compiler/rustc_session/src/errors.rs

@@ -485,6 +485,16 @@ pub(crate) struct FunctionReturnRequiresX86OrX8664;
 #[diag(session_function_return_thunk_extern_requires_non_large_code_model)]
 pub(crate) struct FunctionReturnThunkExternRequiresNonLargeCodeModel;
 
+#[derive(Diagnostic)]
+#[diag(session_unsupported_regparm)]
+pub(crate) struct UnsupportedRegparm {
+    pub(crate) regparm: u32,
+}
+
+#[derive(Diagnostic)]
+#[diag(session_unsupported_regparm_arch)]
+pub(crate) struct UnsupportedRegparmArch;
+
 #[derive(Diagnostic)]
 #[diag(session_failed_to_create_profiler)]
 pub(crate) struct FailedToCreateProfiler {