rust: processor: 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-v2-13-51da5f454a67@google.com

rust/helpers/processor.c

@@ -2,7 +2,7 @@
 
 #include <linux/processor.h>
 
-void rust_helper_cpu_relax(void)
+__rust_helper void rust_helper_cpu_relax(void)
 {
 	cpu_relax();
 }