[fuzzer] Don't hard-code page size in FuzzerUtil.h

Don't hard code the page in FuzzerUtil.h, this breaks on
e.g. LoongArch which defaults to a 16KiB page size.

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D140607

GitOrigin-RevId: a2b677e8153758997a9043360cf51333eecc3c44
2 files changed