lib: make test_bitops compilable into the kernel image

The test now is limited to be compiled as a module. There's no technical
reason for it. Now that the test bears some performance benchmarks, it
would be reasonable to run it at kernel load time, before userspace
starts, to reduce possible jitter.

Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>