atomic: Add atomic_load/store_bool()

[markj]

Summary

For plain accesses, I can see no reason not to provide them. They will
be useful for jhb's re-import of if_wg.

[kib]

sys/sys/atomic_common.h

42

Why #ifdef _KERNEL? Using _Bool, you can do it for userspace, I think. Also, this is a macro, so type is dormant unless the macro is actually used.

[markj]

Drop _KERNEL guards, use _Bool instead.