[scudo][standalone] Introduce the Primary(s) and LocalCache

Summary:
This CL introduces the 32 & 64-bit primary allocators, and associated
Local Cache. While the general idea is mostly similar to what exists
in sanitizer_common, it departs from the original code somewhat
significantly:
- the 64-bit primary no longer uses a free array at the end of a region
  but uses batches of free blocks in region 0, allowing for a
  convergence with the 32-bit primary behavior;
- as a result, there is only one (templated) local cache type for both
  primary allocators, and memory reclaiming can be implemented similarly
  for the 32-bit & 64-bit platforms;
- 64-bit primary regions are handled a bit differently: we do not
  reserve 4TB of memory that we split, but reserve `NumClasses *
  2^RegionSizeLog`, each region being offseted by a random number of
  pages from its computed base. A side effect of this is that the 64-bit
  primary works on 32-bit platform (I don't think we want to encourage
  it but it's an interesting side effect);

Reviewers: vitalybuka, eugenis, morehouse, hctim

Reviewed By: morehouse

Subscribers: srhines, mgorny, delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D61745

llvm-svn: 361159