Start moving stack-related stuff out of thread.h.

This was in preparation for implementing "SmartFrame", which is back on hold
while I finish finalization.

Change-Id: I006c9cfbc34520b9b0a72972611036d908a1d84b
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 69580cc..ae713d0 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -142,6 +142,7 @@
 	src/runtime.cc \
 	src/signal_catcher.cc \
 	src/space.cc \
+	src/stack.cc \
 	src/stringpiece.cc \
 	src/stringprintf.cc \
 	src/stub_arm.cc \