Fix failing allow_user_segv.cc test on OS X 10.10

The current ASan testcase Posix/allow_user_segv.cc expects SIGBUS to be triggered on 32-bit Darwin. This has apparently changed on 10.10 to trigger SIGSEGV instead, just as on 64-bit. Let's just install handlers for both SIGSEGV and SIGBUS instead of #ifdef'ing.

Reviewed at http://reviews.llvm.org/D6121