clean up a few stray c_stdout's in gc debug code

lib/system/gc.nim

@@ -51,6 +51,7 @@ type
     waMarkGlobal,    # part of the backup/debug mark&sweep
     waMarkPrecise,   # part of the backup/debug mark&sweep
     waZctDecRef, waPush
+    #, waDebug
 
   Finalizer {.compilerproc.} = proc (self: pointer) {.nimcall, benign.}
     # A ref type can have a finalizer that is called before the object's
@@ -654,7 +655,7 @@ when logGC:
     else:
       writeCell("cell {", s)
       forAllChildren(s, waDebug)
-      c_fprintf(c_stdout, "}\n")
+      c_fprintf(stdout, "}\n")
 
 proc doOperation(p: pointer, op: WalkOp) =
   if p == nil: return
@@ -665,7 +666,7 @@ proc doOperation(p: pointer, op: WalkOp) =
   case op
   of waZctDecRef:
     #if not isAllocatedPtr(gch.region, c):
-    #  c_fprintf(c_stdout, "[GC] decref bug: %p", c)
+    #  c_fprintf(stdout, "[GC] decref bug: %p", c)
     gcAssert(isAllocatedPtr(gch.region, c), "decRef: waZctDecRef")
     gcAssert(c.refcount >=% rcIncrement, "doOperation 2")
     #c.refcount = c.refcount -% rcIncrement
@@ -828,7 +829,7 @@ proc collectCTBody(gch: var GcHeap) =
     gch.stat.maxPause = max(gch.stat.maxPause, duration)
     when defined(reportMissedDeadlines):
       if gch.maxPause > 0 and duration > gch.maxPause:
-        c_fprintf(c_stdout, "[GC] missed deadline: %ld\n", duration)
+        c_fprintf(stdout, "[GC] missed deadline: %ld\n", duration)
 
 when defined(nimCoroutines):
   proc currentStackSizes(): int =