Read /proc/stat to count online cpus is not correct for all android
kernels. Change to reading /sys/devices/system/cpu/online instead.
Bug: 24376925
Change-Id: I3785a6c7aa15a467022a9a261b457194d688fb38
It turns out that everyone's still getting PAGE_SIZE from <sys/user.h> via
<sys/ucontext.h> via <signal.h> anyway.
glibc has PAGE_SIZE in <sys/user.h> rather than <limits.h> so this part is
good. The bad part is that we have such wide transitive inclusion of
<sys/user.h>!
Bug: http://b/22735893
Change-Id: I363adffe4a27b4ca1eedf695ea621f5dd2d5ca10
