[scudo] Check if MADV_DONTNEED zeroes memory

QEMU just ignores MADV_DONTNEED
https://github.com/qemu/qemu/blob/b1cffefa1b163bce9aebc3416f562c1d3886eeaa/linux-user/syscall.c#L11941

Depends on D100998.

Differential Revision: https://reviews.llvm.org/D101031