Add cryptoutil::zero_memory. The ARM implementation is untested.

This adds an assembly-implemented secure-zero-memory function. I am not sure that assembly is the right way to approach this, but if you think it is here is an implementation that at least works on x86.

The possibly-better way to approach this would be calling out to operating-system functions to do this, but there are operating systems that don't seem to have such a function available. So, that would mean having to implement this across all architectures anyway. 

I don't have any ARM device, so I don't know if the test will pass on one.
Do you mean for this to be a pub function?

I think the `*const u8` should be `*mut u8` since the memory pointed to be `xsp` will be modified.