zpu: wip - very crude stack slot pass

zpu: wip - very crude stack slot pass

This is soooooo crude, but it has a stack slot pass
that converst registers into stack slots.

clang   -ccc-host-triple zpu-none-none test3.c -S

cat test3.s
	.file	"test3.c"
	.text
	.globl	foo
	.align	2
	.type	foo,@function
	load 0 %sp
	storestackslot %sp 52, %sp
	load 0 %sp
	storestackslot %sp 56, %sp
	loadstackslot 0, 0 %sp
	add %sp 0 %sp
	storestackslot %sp 60, %sp
	storereg %sp c
$tmp0:
	.size	foo, ($tmp0)-foo

	.type	b,@object
	.comm	b,4,4
	.type	c,@object
	.comm	c,4,4

---

int b;
int c;
void foo(void)
{
  int d = b;
  int e = c;
  int f;
  f = d + e;
  c = f;
}