runtime: move vdso_linux_amd64.go to vdso_linux.go

This is a preparation step for adding vDSO support on linux/386.

In a follow-on change, the vDSO ELF symbol lookup code in this
file will be refactored so it can be used on multiple architectures.

First, move the file to an architecture-neutral file name so that
the change history is preserved. Build tags are added so that the
build behaves as it did before.

vdso_linux_amd64.go will be recreated later, just containing the
amd64 specifics.

If the move and refactor were combined in a single change, then the
history to date would be lost because git would see the existing code
as a new file.

Change-Id: Iddb5da0d7faf141fd7cc835fe6a80c80153897e9
Reviewed-on: https://go-review.googlesource.com/69710
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/vdso_linux_amd64.go → src/runtime/vdso_linux.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build linux
+// +build amd64
+
 package runtime
 
 import "unsafe"