[scudo] Fix MallocIterateBoundary on 32 bit.

On Android, the 32 bit value of 8 for BlockDelta results in a
failure because a valid pointer can never be found.

Change the code to always use 16 which passes on both 32 bit
and 64 bit.

Verified that re-introducing the old bug causes the test to fail
on both 32 bit and 64 with the new BlockDelta value.

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D152767