[scudo] Temporariy dispatch region from `RegionBeg`

In general, a region is located from region base and has size
`RegionSize`. However, some platforms may not support mapping from
region base. Before we have each platform implements their specific
MemMap to handle the offset. Temporarily dispatch the region from
`RegionBeg` instead.

Reviewed By: cferris, fabio-d

Differential Revision: https://reviews.llvm.org/D147792