internal/syscall/unix: add randomTrap for m68k

Patch by Andrwas Schwab.
    
    Reviewed-on: https://go-review.googlesource.com/38456

From-SVN: r246410

libgo/go/internal/syscall/unix/getrandom_linux_m68k.go

@@ -0,0 +1,9 @@
+// Copyright 2017 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.
+
+package unix
+
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
+const randomTrap uintptr = 352