Disable Android 4.0, add arch detection.

Android 4.0, it turns out, has a different echo builtin than the other androids.
Until we can figure out how to drop a payload on a 4.0 shell, we cannot support it.

Arch detection allows mips/x86/arm ndkstagers to work, unfortunately
x86 ndkstager was not working, so it is disabled for now.