cgo: enable cgo on OpenBSD

src/pkg/runtime/cgo/gcc_openbsd_386.c

 // Copyright 2009 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include <sys/types.h>
 #include <dlfcn.h>
 #include <pthread.h>
 #include <signal.h>
 #include <string.h>
 #include <errno.h>
 #include "libcgo.h"
 
 static void* threadentry(void*);
 
-#define TCB_SIZE 16
+// TCB_SIZE is sizeof(struct thread_control_block),·
+// defined in /usr/src/lib/librthread/tcb.h
+#define TCB_SIZE (4 * sizeof(void *))
 #define TLS_SIZE (2 * sizeof(void *))
 
 void *__get_tcb(void);
 void __set_tcb(void *);
 
 static void
 tcb_fixup(int mainthread)
 {
         void *newtcb, *oldtcb;
 
@@ skipping 128 matching lines @@
          */
         asm volatile (
                 "movl %0, %%gs:-8\n"    // MOVL g, -8(GS)
                 "movl %1, %%gs:-4\n"    // MOVL m, -4(GS)
                 :: "r"(ts.g), "r"(ts.m)
         );
 
         crosscall_386(ts.fn);
         return nil;
 }