runtime: make go-nosys.c compile with !HAVE_SYSCALL

Reviewed-on: https://go-review.googlesource.com/59910

From-SVN: r251435

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-db8e3801bf8508656606d6e465c76cdc6e9a9eb7
+a28f1d8aa306bdb5166af1f087e5f5027ce51d6d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-nosys.c

@@ -506,7 +506,7 @@ strerror_r (int errnum, char *buf, size_t buflen)
 
 #ifndef HAVE_SYSCALL
 int
-syscall(int number, ...)
+syscall(int number __attribute__ ((unused)), ...)
 {
   errno = ENOSYS;
   return -1;