[test] Fix page address logic in clear_cache_test

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
4096.

Differential Revision: https://reviews.llvm.org/D28849

llvm-svn: 292729