rename globals cpu and proc to _cpu and _proc

On x86-64 the "global register variable" hack does not work, so
we're going to need macros or other trickery to access these.

Unfortunately they have the same names as their structs so that
causes all kinds of badness.  Renaming the structs to cpu_t and
proc_t or the like didn't avoid more collisions since cpu and
proc are used for local variable names in various places too.

I discarded CPU and PROC or Cpu and Proc as too weird-looking
in a codebase that is almost exclusively lowercase, and settled
on _cpu and _proc as non-colliding but distinct.  They also draw
some attention to the use of these, which makes sense because
they aren't regular variables or even regular globals.