[scudo][standalone] Introduce platform specific code & mutexes

Summary:
This CL adds the platform specific code for Fuchsia, Linux & Android,
as well as some tests related to those (more tests to come later).
While some of it is pretty much a straight port of the existing scudo &
sanitizer_common code, the memory mapping functions have been reworked
a bit to fit the limited usage scenario that Scudo has for them.

For Fuchsia, I can now track the Vmar/Vmo pair for memory mappings if
there is an intent to grow or decommit some mapping (that will be
useful for the Primary).

Reviewers: eugenis, vitalybuka, mcgrathr, phosek, flowerhack, morehouse, dmmoore415

Reviewed By: vitalybuka, morehouse

Subscribers: kcc, dvyukov, srhines, mgorny, delcypher, jfb, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58184

llvm-svn: 354895