This is a follow up patch to the series I sent recently that cleans up
KASAN_SHADOW_SCALE_SHIFT usage (which value was hardcoded and scattered
all over the code). This fixes the one place that I forgot to fix.

The change is purely aesthetical, instead of hardcoding the value for
KASAN_SHADOW_SCALE_SHIFT in arch/arm64/Makefile, an appropriate variable
is declared and used.
Signed-off-by: NAndrey Konovalov <andreyknvl@google.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>