rust: helpers: Move #define __rust_helper out of atomic.c

rust: helpers: Move #define __rust_helper out of atomic.c

In order to support inline helpers [1], we need to have __rust_helper
defined for all helper files. Current we are lucky that atomic.c is the
first file in helpers.c, but this is fragile. Thus, move it to
helpers.c.

[boqun: Reword the commit message and apply file hash changes]

Link: https://lore.kernel.org/r/20260105-define-rust-helper-v2-0-51da5f454a67@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20260107-move-rust_helper-define-v1-1-4109d58ef275@google.com