debuggerd: a few generic improvements

This one makes dump_memory reasonably architecture-agnostic so it is
possible to share the code between architectures.

It also includes a few small improvements in tombstone.cpp.

Change-Id: Ib8a9599bfa420b41e80207988e87aee1b9d79541
Signed-off-by: Kévin PETIT <kevin.petit@arm.com>