Remove bad memory readability assumptions

The page before the first one allocated might actually be readable.
Especially if the implementation uses that page for storing metadata.
Hard-code page size on iOS/ARM64

The iOS 8.1 kernel reports 4096 instead of 16384. Either a bug in the
latest kernel, or in the Pangu jailbreak.
Assume 4096 byte alignment of Mach-O images in memory