compiler.h: Enforce that READ_ONCE_NOCHECK() access size is sizeof(long)

READ_ONCE_NOCHECK() unconditionally performs a sizeof(long)-sized access,
so enforce that the size of the pointed-to object that we are loading
from is the same size as 'long'.

Reported-by: Marco Elver <elver@google.com>
Signed-off-by: Will Deacon <will@kernel.org>