[hwasan] Remove system allocator fallback.

Summary:
This has been an experiment with late malloc interposition, made
possible by a non-standard feature of the Android dynamic loader.

Reviewers: pcc, mmalcomson

Subscribers: srhines, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69199

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@375296 91177308-0d34-0410-b5e6-96231b3b80d8