[scudo][standalone] Add the memory reclaiming mechanism

Summary:
This CL implements the memory reclaiming function `releaseFreeMemoryToOS`
and its associated classes. Most of this code was originally written by
Aleksey for the Primary64 in sanitizer_common, and I made some changes to
be able to implement 32-bit reclaiming as well. The code has be restructured
a bit to accomodate for freelist of batches instead of the freearray used
in the current sanitizer_common code.

Reviewers: eugenis, vitalybuka, morehouse, hctim

Reviewed By: vitalybuka

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D61214

llvm-svn: 359567