cmd/[568]l/obj.c: NULL is not recognised in Plan 9 build, use nil instead.

Fixes #5591.

R=golang-dev, dave, minux.ma, cshapiro
CC=carl shapiro <cshapiro, golang-dev
https://golang.org/cl/9839046

src/cmd/5l/obj.c

@@ -647,7 +647,7 @@ loop:
 	case APTRS:
 		if(skip)
 			goto casedef;
-		if(cursym->nptrs == -1 || cursym->ptrs == NULL) {
+		if(cursym->nptrs == -1 || cursym->ptrs == nil) {
 			diag("ldobj1: pointer map data provided for %s without a definition", cursym->name);
 			errorexit();
 		}

src/cmd/6l/obj.c

@@ -636,7 +636,7 @@ loop:
 	case APTRS:
 		if(skip)
 			goto casdef;
-		if(cursym->nptrs == -1 || cursym->ptrs == NULL) {
+		if(cursym->nptrs == -1 || cursym->ptrs == nil) {
 			diag("ldobj1: pointer map data provided for %s without a definition", cursym->name);
 			errorexit();
 		}

src/cmd/8l/obj.c

@@ -646,7 +646,7 @@ loop:
 	case APTRS:
 		if(skip)
 			goto casdef;
-		if(cursym->nptrs == -1 || cursym->ptrs == NULL) {
+		if(cursym->nptrs == -1 || cursym->ptrs == nil) {
 			diag("ldobj1: pointer map data provided for %s without a definition", cursym->name);
 			errorexit();
 		}