[Scudo] [GWP-ASan] Add GWP-ASan to Scudo Standalone.

Summary:
Adds GWP-ASan to Scudo standalone. Default parameters are pulled across from the
GWP-ASan build. No backtrace support as of yet.

Reviewers: cryptoad, eugenis, pcc

Reviewed By: cryptoad

Subscribers: merge_guards_bot, mgorny, #sanitizers, llvm-commits, cferris, vlad.tsyrklevich, pcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71229