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

This reverts commit a2b677e8153758997a9043360cf51333eecc3c44.

reverting

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

because <sys/auxv.h> and getauxval() are not available on macOS;
this change is breaking the mac CI bots.

GitOrigin-RevId: cb9f2de2e80292c0445c99be754fcc78b94b0f16