kasan, arm64: Add KASAN light mode

Architectures supported by KASAN HW can provide a light mode of
execution. On an MTE enabled arm64 hw for example this can be identified
with the asynch mode of execution. If an async exception occurs, the
arm64 core updates a register which is asynchronously detected the next
time in which the kernel is accessed.

KASAN requires a specific mode of execution to make use of this hw feature.

Add KASAN HW light execution mode.

Note: This patch adds the KASAN_ARG_MODE_LIGHT config option and the
"light" kernel command line option to enable the described feature.
This patch introduces the kasan_def.h header to make easier to propagate
the relevant enumerations to the architectural code.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>