- make proper phread.h header for all architectures

Index: lib/std/private/threadtypes.nim
--- lib/std/private/threadtypes.nim.orig
+++ lib/std/private/threadtypes.nim
@@ -77,7 +77,7 @@ else:
       Pthread_attr* {.importc: "pthread_attr_t",
                     header: "<sys/types.h>".} = object
         abi: array[56 div sizeof(clong), clong]
-  elif defined(openbsd) and defined(amd64):
+  elif defined(openbsd):
     type
       SysThread* {.importc: "pthread_t", header: "<pthread.h>".} = object
       Pthread_attr* {.importc: "pthread_attr_t",
