[scudo] Adding a public Scudo interface

Summary:
The first and only function to start with allows to set the soft or hard RSS
limit at runtime. Add associated tests.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: mgorny, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D41128

llvm-svn: 320611