rust: cpu: Add __rust_helper to helpers

This is needed to inline these helpers into Rust code.

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20260105-define-rust-helper-v2-6-51da5f454a67@google.com

Author: Darksonn

rust/helpers/cpu.c

@@ -2,7 +2,7 @@
 
 #include <linux/smp.h>
 
-unsigned int rust_helper_raw_smp_processor_id(void)
+__rust_helper unsigned int rust_helper_raw_smp_processor_id(void)
 {
 	return raw_smp_processor_id();
 }