[GWP-ASan] Initial build files, implementation of PRNG [1].

Summary:
See D60593 for further information.
This patch slices off the PRNG implementation and the initial build files for GWP-ASan.

Reviewers: vlad.tsyrklevich, morehouse, vitalybuka

Reviewed By: morehouse

Subscribers: srhines, kubamracek, mgorny, #sanitizers, llvm-commits, cryptoad, eugenis

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61867

llvm-svn: 360710