[GWP-ASan] [Scudo] Add GWP-ASan backtrace for alloc/free to Scudo.

Summary:
Adds allocation and deallocation stack trace support to Scudo. The
default provided backtrace library for GWP-ASan is supplied by the libc
unwinder, and is suitable for production variants of Scudo. If Scudo in future
has its own unwinder, it may choose to use its own over the generic unwinder
instead.

Reviewers: cryptoad

Reviewed By: cryptoad

Subscribers: kubamracek, mgorny, #sanitizers, llvm-commits, morehouse, vlad.tsyrklevich, eugenis

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D64085

llvm-svn: 364966