rust: helpers: Add i8/i16 atomic xchg_relaxed helpers

rust: helpers: Add i8/i16 atomic xchg_relaxed helpers

Add i8/i16 atomic xchg_relaxed helpers that call xchg_relaxed() macro
implementing atomic xchg_relaxed using architecture-specific
instructions.

[boqun: Use xchg_relaxed() instead of raw_xchg_relaxed()]

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20251223062140.938325-5-fujita.tomonori@gmail.com