[asan] Enable back halt_on_error-torture.cc disabled on PowerPC.

Summary:
D33521 addressed a memory ordering issue in BlockingMutex, which seems
to be the cause of a flakiness of a few ASan tests on PowerPC.

Reviewers: eugenis

Subscribers: kubamracek, nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D33611

llvm-svn: 304045