mem: Add magic field to detect memory corruption

mem: Add magic field to detect memory corruption

Very little checking is performed on the validity of freelist
metadata. Add a magic field to struct arena_header which can be used
to detect memory corruption or whether a user tries to free something
that isn't a malloc pointer.

This feature can be enabled with -DDEBUG_MALLOC. Turn it on for
development in mk/devel.mk.

Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>