[scudo][standalone] Adding a stats class

Summary:
This adds simple local & global stats classes to be used by the Primary
and Secondary, and associated test. Note that we don't need the strict
atomicity of the addition & subtraction (as is in sanitizer_common) so
we just use load & store.

Reviewers: morehouse, vitalybuka, eugenis, flowerhack, dmmoore415

Reviewed By: morehouse, vitalybuka

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59031

llvm-svn: 355643