rust: helpers: Add i8/i16 atomic xchg_acquire helpers

rust: helpers: Add i8/i16 atomic xchg_acquire helpers

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

[boqun: Use xchg_acquire() instead of raw_xchg_acquire()]

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/20251223062140.938325-3-fujita.tomonori@gmail.com