ART: Pass ucontext to Backtrace in Stack Dump

In case of an unexpected signal on the host we dump the thread stack
ourselves. We have to pass the context given to the signal handler,
as the signal handler is run on an alternate stack. Otherwise
libbacktrace can't dump the actual faulty part.

Bug: 18933933
Change-Id: Id2710d2fd44b7c3b3335973a9288979a5793638b