selftests: cachestat: use proper syscall number macro

At the moment the cachestat syscall number is hard coded into the test
source code.
Remove that and replace it with the proper __NR_cachestat macro.
That ensures compatibility should other architectures pick a different
number.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>