Add memory ordering constraint, convert to C11 atomics

Add an ordering constraint/fence to __system_property_serial.
This slows down a read on a Nexus 5 from about 50 to about 70 ns,
but avoids the possibility of seeing an inconsistent property value.
Use C11 atomic operations where easy and appropriate.
This code remains not fully C++11 memory model conformant, but
I would now expect the generated code to now be correct with current compilers.

Bug:14970171
Change-Id: I0891ff1d0f914ae5c3857e3d76b6a7c8a4a07d83