rust: sync: atomic: Add performance-optimal Flag type for atomic booleans

rust: sync: atomic: Add performance-optimal Flag type for atomic booleans

Add AtomicFlag type for boolean flags.

Document when AtomicFlag is generally preferable to Atomic<bool>: in
particular, when RMW operations such as xchg()/cmpxchg() may be used
and minimizing memory usage is not the top priority. On some
architectures without byte-sized RMW instructions, Atomic<bool> can be
slower for RMW operations.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Signed-off-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260129122622.3896144-2-tomo@aliasing.net
Link: https://patch.msgid.link/20260303201701.12204-9-boqun@kernel.org