Updated the README to account for clang
OS now prints an H when main exits

In the lower-right corner. This is meant as an exercise to demonstrate
to myself that assembly is not my strong point. That's what practice
(like this) is for, though. In any case, the real reason is that I think
that having that H there will help distinguish between an infinite loop
and a halt.
Removed early test code

While I am generally of the opinion that it is hard to begin testing too early, I
think this might actually be too early. Once I have more code and an
idea of what would belong in test_kernel_early, I might add it back.