[scudo] Scudo thread specific data refactor, part 3

Summary:
Previous parts: D38139, D38183.

In this part of the refactor, we abstract the Linux vs Android TSD dissociation
in favor of a Exclusive vs Shared one, allowing for easier platform introduction
and configuration.

Most of this change consist of shuffling the files around to reflect the new
organization.

We introduce `scudo_platform.h` where platform specific definition lie. This
involves the TSD model and the platform specific allocator parameters. In an
upcoming CL, those will be configurable via defines, but we currently stick
with conservative defaults.

Reviewers: alekseyshl, dvyukov

Reviewed By: alekseyshl, dvyukov

Subscribers: srhines, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D38244

llvm-svn: 314224