forsyth: static and extern volatile deoptimization

Reference: /n/atom/patch/applied/ccexternvolatile
Date: Sun Aug 16 01:35:44 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/6c/reg.c	Sun Aug 16 01:35:34 2015
+++ /sys/src/cmd/6c/reg.c	Sun Aug 16 01:35:36 2015
@@ -732,6 +732,7 @@
 	r->regu |= doregbits(t);
 	r->regu |= doregbits(a->index);
 
+	s = a->sym;
 	switch(t) {
 	default:
 		goto none;
@@ -744,12 +745,13 @@
 		goto none;
 	case D_EXTERN:
 	case D_STATIC:
+		if(s->type != nil && s->type->garb & GVOLATILE)
+			goto none;
 	case D_PARAM:
 	case D_AUTO:
 		n = t;
 		break;
 	}
-	s = a->sym;
 	if(s == S)
 		goto none;
 	if(s->name[0] == '.')