Tweaked and combined the compiler checks

I was thinking of removing them, but I figure that since it probably
won't work without the cross-compiler anyway, I might as well leave it
in to encourage people to go through the process of setting up the
cross-compiler.
Pulled out tty.c from kernel.c

And updated the Tupfile to match. As before, I'm still following along
the meatier skeleton, although I'm thinking I might start diverging
soonish.
Moved kernel.c to the arch dir

Where it belongs, and added foreach to my Tupfile to fix the exposed
bug(?)
Added kernel_early for setup stuff

In case I eventually want global constructors, I split this stuff out. I
also trimmed the comments back a lot, since this isn't the place for it.