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
(cherry picked from commit e34c71ce21dbc9fac3427df887e90e1311b6fa8e)

Change-Id: Ib1105a3470064d98843bd46f32d4fadb1d20e416
diff --git a/runtime/thread_linux.cc b/runtime/thread_linux.cc
index 0731f30..9d54eba 100644
--- a/runtime/thread_linux.cc
+++ b/runtime/thread_linux.cc
@@ -36,11 +36,11 @@
 }
 
 // The default SIGSTKSZ on linux is 8K.  If we do any logging in a signal
-// handler this is too small.  We allocate 16K instead or the minimum signal
-// stack size.
+// handler or do a stack unwind, this is too small.  We allocate 32K
+// instead of the minimum signal stack size.
 // TODO: We shouldn't do logging (with locks) in signal handlers.
 static constexpr int kHostAltSigStackSize =
-    16 * KB < MINSIGSTKSZ ? MINSIGSTKSZ : 16 * KB;
+    32 * KB < MINSIGSTKSZ ? MINSIGSTKSZ : 32 * KB;
 
 void Thread::SetUpAlternateSignalStack() {
   // Create and set an alternate signal stack.