Modify the behavior of thread suspend shootouts.

The thread doing the suspension doesn't attempt to suspend the other thread
unless it knows another thread isn't trying to suspend it. Use the suspend
count, and its lock, for this purpose.
Re-enable ThreadStress test.
Bug: 15446488

Change-Id: Idd34410c7b89d8abd6973e5699a15ca699472c78
diff --git a/build/Android.common_build.mk b/build/Android.common_build.mk
index 7e58f5c..a221cfc 100644
--- a/build/Android.common_build.mk
+++ b/build/Android.common_build.mk
@@ -173,7 +173,9 @@
 
 
 ifeq ($(ART_HOST_CLANG),true)
-  ART_HOST_CFLAGS += $(art_clang_cflags)
+  # Bug: 15446488. We don't omit the frame pointer to work around
+  # clang/libunwind bugs that cause SEGVs in run-test-004-ThreadStress.
+  ART_HOST_CFLAGS += $(art_clang_cflags) -fno-omit-frame-pointer
 else
   ART_HOST_CFLAGS += $(art_gcc_cflags)
 endif