kasan: separate metadata_fetch_row for each mode

This is a preparatory commit for the upcoming addition of a new hardware
tag-based (MTE-based) KASAN mode.

Rework print_memory_metadata() to make it agnostic with regard to the
way metadata is stored. Allow providing a separate metadata_fetch_row()
implementation for each KASAN mode. Hardware tag-based KASAN will provide
its own implementation that doesn't use shadow memory.

No functional changes for software modes.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Marco Elver <elver@google.com>
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Change-Id: I5b0ed1d079ea776e620beca6a529a861e7dced95