Move to exception.S. This contains the vector to handle the exceptions
we can configure at EL1.
Load the exception vector on boot. We can now use brk to signal mi_startup
returned as it should never happen.

While here clean the stye a little.
The brk instructions has been tested to work, remove the comment.
Add the needed autoconf functions based on the mips version.
Add useful defines to the arm64 machine/asm.h
Switch to an asm version of swtch.S
Move cpu_throw and cpu_switch to swtch.S.
Add a minimal copy of cpu_startup
Set the size on the END macro
Add more useful offsets
Implement cpu_switch. It's missing things like cache flushing and SMP
support, however neigher of these are needed at this point.