internal/syscall/unix: add randomTrap for sh/shbe

    
    CL 84555 added support for the SuperH architecture, but didn't add the
    randomTrap definition to be used for the getrandom syscall on Linux.
    Add it now.
    
    Reviewed-on: https://go-review.googlesource.com/90535

From-SVN: r257171

gcc/go/gofrontend/MERGE

-6517e6731aeb4512d12c341c7111959a44547ba0
+bbce8a9af264b25c5f70bafb2ce95d4fed158d68
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/syscall/unix/getrandom_linux_shx.go

+// Copyright 2018 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.
+
+// +build sh shbe
+
+package unix
+
+// Linux getrandom system call number.
+// See GetRandom in getrandom_linux.go.
+const randomTrap uintptr = 373