Enable mprotect-based VDB on Serenity OS

[ivmai]

As tested and reported by @linusg in issue #688, MPROTECT_VDB causes unexpected null pointer dereference when the incremental collection is enabled. A workaround (in PR #689) is prepared to deal with the failure (e2e6fe9).

This task (a low-priority one) is to find the root cause of the mentioned failure and fix it making MPROTECT_VDB defined again.