GCs support ForeignCells

lib/system/syslocks.nim

@@ -9,6 +9,8 @@
 
 # Low level system locks and condition vars.
 
+{.push stackTrace: off.}
+
 when defined(Windows):
   type
     Handle = int
@@ -118,3 +120,5 @@ else:
 
   proc deinitSysCond(cond: var SysCond) {.noSideEffect,
     importc: "pthread_cond_destroy", header: "<pthread.h>".}
+
+{.pop.}