[asan] Add fallback for Thumb after r350139

This reverts r350806 which marked some tests as UNSUPPORTED on ARM and
instead reintroduces the old code path only for Thumb, since that seems
to be the only target that broke.

It would still be nice to find the root cause of the breakage, but with
the branch point for LLVM 8.0 scheduled for next week it's better to put
things in a stable state while we investigate.

Differential Revision: https://reviews.llvm.org/D56594

llvm-svn: 351040