This adds option KCSAN_ASSUME_PLAIN_WRITES_ATOMIC. If enabled, plain
aligned writes up to word size are assumed to be atomic, and also not
subject to other unsafe compiler optimizations resulting in data races.

This option has been enabled by default to reflect current kernel-wide
preferences.
Signed-off-by: NMarco Elver <elver@google.com>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>