Fix the logic used to calculate page boundaries in clear_cache_test to
use correct masks -- e.g. -4096 rather than -4095. The latter gives
incorrect result since:
-4095 -> 0xfffff001 -4096 -> 0xfffff000 (== ~4095)
The issue went unnoticed so far because the array alignment caused
the last bit not to be set. However, on 32-bit x86 no such alignment is
enforced and the wrong page address caused the test to fail.
Furthermore, obtain the page size from the system instead of hardcoding