runtime: work around 5c bug in GC code.

5c miscompiles *p++ = struct_literal.

R=dave, golang-dev
CC=golang-dev
https://golang.org/cl/7065069

src/pkg/runtime/mgc0.c

@@ -338,7 +338,8 @@ flushptrbuf(PtrTarget *ptrbuf, PtrTarget **ptrbufpos, Obj **_wp, Workbuf **_wbuf
 			if((bits & (bitAllocated|bitMarked)) != bitAllocated)
 				continue;
 
-			*bitbufpos++ = (BitTarget){obj, ti, bitp, shift};
+			*bitbufpos = (BitTarget){obj, ti, bitp, shift};
+			bitbufpos++;
 		}
 
 		runtime·lock(&lock);
@@ -541,7 +542,8 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 			
 			// iface->tab
 			if((void*)iface->tab >= arena_start && (void*)iface->tab < arena_used) {
-				*ptrbufpos++ = (PtrTarget){iface->tab, (uintptr)itabtype->gc};
+				*ptrbufpos = (PtrTarget){iface->tab, (uintptr)itabtype->gc};
+				ptrbufpos++;
 				if(ptrbufpos == ptrbuf_end)
 					flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj, bitbuf);
 			}
@@ -568,7 +570,8 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 				stack_top.b += PtrSize;
 				obj = *(byte**)i;
 				if(obj >= arena_start && obj < arena_used) {
-					*ptrbufpos++ = (PtrTarget){obj, 0};
+					*ptrbufpos = (PtrTarget){obj, 0};
+					ptrbufpos++;
 					if(ptrbufpos == ptrbuf_end)
 						flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj, bitbuf);
 				}
@@ -654,7 +657,8 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 		}
 
 		if(obj >= arena_start && obj < arena_used) {
-			*ptrbufpos++ = (PtrTarget){obj, objti};
+			*ptrbufpos = (PtrTarget){obj, objti};
+			ptrbufpos++;
 			if(ptrbufpos == ptrbuf_end)
 				flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj, bitbuf);
 		}