8g: use uintptr for local pc

Fixes #2478.

R=ken2
CC=golang-dev
https://golang.org/cl/5593051

src/cmd/8g/gsubr.c

@@ -783,7 +783,7 @@ ginit(void)
 		reg[resvd[i]]++;
 }
 
-ulong regpc[D_NONE];
+uintptr regpc[D_NONE];
 
 void
 gclean(void)
@@ -871,7 +871,7 @@ out:
 	if (i == D_SP)
 		print("alloc SP\n");
 	if(reg[i] == 0) {
-		regpc[i] = (ulong)getcallerpc(&n);
+		regpc[i] = (uintptr)getcallerpc(&n);
 		if(i == D_AX || i == D_CX || i == D_DX || i == D_SP) {
 			dump("regalloc-o", o);
 			fatal("regalloc %R", i);