use a register variable to get at the curcpu pointer in %g7

use a register variable to get at the curcpu pointer in %g7

this lets us define curcpu(), cpu_number(), etc as derefs of this
variable rather than having to write asm to copy it into another
register for the same derefs.

this provides shorter inlined code that wastes less registers.

ok kettenis@ who notes we use this these on other archs already