Replace shadow space zero-out by madvise at mmap

After D88686, munmap uses MADV_DONTNEED to ensure zero-out before the
next access. Because the entire shadow space is created by MAP_PRIVATE
and MAP_ANONYMOUS, the first access is also on zero-filled values.

So it is fine to not zero-out data, but use madvise(MADV_DONTNEED) at
mmap. This reduces runtime
overhead.

Reviewed-by: morehouse

Differential Revision: https://reviews.llvm.org/D88755