internal/syscall/unix: eliminate non-trivial randomTrap initializer

While here, enable getrandom on arm64 too (using the value found in
include/uapi/asm-generic/unistd.h, which seems to match up with other
GOARCH=arm64 syscall numbers).

Updates #10848.

Change-Id: I5ab36ccf6ee8d5cc6f0e1a61d09f0da7410288b9
Reviewed-on: https://go-review.googlesource.com/16662
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/syscall/unix/getrandom_linux.go

@@ -5,20 +5,11 @@
 package unix
 
 import (
-	"runtime"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
 )
 
-var randomTrap = map[string]uintptr{
-	"386":     355,
-	"amd64":   318,
-	"arm":     384,
-	"ppc64":   359,
-	"ppc64le": 359,
-}[runtime.GOARCH]
-
 var randomUnsupported int32 // atomic
 
 // GetRandomFlag is a flag supported by the getrandom system call.

src/internal/syscall/unix/getrandom_linux_386.go

+// Copyright 2014 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
+
+const randomTrap uintptr = 355

src/internal/syscall/unix/getrandom_linux_amd64.go

+// Copyright 2014 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
+
+const randomTrap uintptr = 318

src/internal/syscall/unix/getrandom_linux_arm.go

+// Copyright 2014 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
+
+const randomTrap uintptr = 384

src/internal/syscall/unix/getrandom_linux_generic.go

+// Copyright 2014 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 arm64
+
+package unix
+
+const randomTrap uintptr = 278

src/internal/syscall/unix/getrandom_linux_ppc64x.go

+// Copyright 2014 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 ppc64 ppc64le
+
+package unix
+
+const randomTrap uintptr = 359