[GWP-ASan] Fixed issue with c++ standard library dependency.

Summary:
Removed dependency on c++ standard library. Some supporting allocators (namely Scudo on Fuchsia, and shortly, scudo standalone) has a hard requirement of no c++stdlib.

This patch updates the build system so that we don't have any c++ stdlib dependencies. It also will conveniently fix a racy build-order bug discrepency between GWP-ASan and libc++.

Reviewers: phosek, morehouse

Reviewed By: phosek, morehouse

Subscribers: kubamracek, mgorny, cryptoad, #sanitizers, llvm-commits, beanz, smeenai, vitalybuka

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D62048

llvm-svn: 360982