syscall: don't assume that WIFCONTINUED is defined

It's not defined on the Hurd.
    
    Reviewed-on: https://go-review.googlesource.com/c/161963

From-SVN: r268828

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-fc8aa5a46433d6ecba9fd1cd0bee4290c314ca06
+6d03c4c8ca320042bd550d44c0f25575c5311ac2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/wait.c

@@ -16,6 +16,10 @@
 #define WCOREDUMP(status) (((status) & 0200) != 0)
 #endif
 
+#ifndef WIFCONTINUED
+#define WIFCONTINUED(x) 0
+#endif
+
 extern _Bool Exited (uint32_t *w)
   __asm__ (GOSYM_PREFIX "syscall.WaitStatus.Exited");