[ASan] Get fake stack code working with GCC 4.8.2.

Summary:
TestCases/Linux/heavy_uar_test.cc was failing on my
PowerPC64 box with GCC 4.8.2, because the compiler recognised
a memset-like loop and turned it into a call to memset, which
got intercepted by __asan_memset, which got upset because it was
being called on an address in high shadow memory.

Use break_optimization to stop the compiler from doing this.

Reviewers: kcc, samsonov

Reviewed By: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6266

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@222572 91177308-0d34-0410-b5e6-96231b3b80d8