mm, kasan: Update kasan docs to indicate arm64 support

KASAN has been supported on arm64 since 39d114dd ("arm64: add KASAN
support"). Update the docs to indicate this.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>