all: build tags: appengine,!gccgo => purego,gc

appengine is obsolete and superseded by purego, and gc is a more
precise tag for files that use gc-syntax assembly.

Change-Id: I716f59de772ebeee4adf4d2a432edf300122cef0
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/269920
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

internal/subtle/aliasing.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !appengine
+// +build !purego
 
 // Package subtle implements functions that are often useful in cryptographic
 // code but require careful thought to use correctly.

internal/subtle/aliasing_purego.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appengine
+// +build purego
 
 // Package subtle implements functions that are often useful in cryptographic
 // code but require careful thought to use correctly.