disable pinToCpu for OSX

lib/system/threads.nim

@@ -463,6 +463,7 @@ else:
     ## pins a thread to a `CPU`:idx:. In other words sets a
     ## thread's `affinity`:idx:. If you don't know what this means, you
     ## shouldn't use this proc.
+    when not defined(macosx):
       var s {.noinit.}: CpuSet
       cpusetZero(s)
       cpusetIncl(cpu.cint, s)