believe it or not, this was working

the macro expansion of "char *cp;" turned into
char *(curproc[cpu()]);  which declares a dynamically
sized array of char* called curproc.

so then &cp == &(curproc[cpu()]) was actually a
stack variable as "expected".  it was one past the
end of the array, but the implicit alloca allocated
more than was necessary.

do not tell me that making cp a #define was a bad idea.
there are worse problems to fix.  more on that later.