[scudo] Initial standalone skeleton check-in

Summary:
This is the initial check-in for the Standalone version of Scudo.

The project is initially going to live in scudo/standalone then will
replace scudo. See http://lists.llvm.org/pipermail/llvm-dev/2019-January/129113.html
for details.

This initial CL is meant to lay out the project structure, of both
code & tests, providing a minimal amount of functionalities, namely
various definitions, some atomic helpers and an intrusive list.
(empty.cc is just here to have a compilation unit, but will go away
in the upcoming CLs).

Initial support is restricted to Linux i386 & x86_64 in make files
and will be extended once things land & work.

We will grow organically from here, adding functionalities in limited
amounts.

Reviewers: morehouse, eugenis, vitalybuka, kcc, mcgrathr, flowerhack

Reviewed By: morehouse, vitalybuka

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D57412

llvm-svn: 353055