selftest: size: Add size test for Linux kernel

This test shows the amount of memory used by the system.
Note that this is dependent on the user-space that is loaded
when this program runs.  Optimally, this program would be
run as the init program itself.

The program is optimized for size itself, to avoid conflating
its own execution with that of the system software.
The code is compiled statically, with no stdlibs. On my x86_64 system,
this results in a statically linked binary of less than 5K.

Signed-off-by: Tim Bird <tim.bird@sonymobile.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>