[asan] Allocator support for Fuchsia

Submitted on behalf of Roland McGrath.

Reviewers: vitalybuka, alekseyshl, kcc

Reviewed By: alekseyshl

Subscribers: srhines, cryptoad, kubamracek, phosek, filcab, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36190

llvm-svn: 309914