Increase alt signal stack to 32K on host.

On host, when there is a crash, the code tries to do an unwind
from the signal handler. Under some conditions this can require
a stack larger than 16K, so increase this value to 32K.

The symptom of this is that you get an empty backtrace running
art tests when there is a crash.

Bug: 21949571
Change-Id: I8306da55e35212a0f349e43a7f2ad546e8da8c76