crypto/internal/fips140/subtle: combine xor_<goarch>.go files

There is not much point in having per architecture files that all contain
the same content. Instead, merge the various xor_<goarch>.go files into a
single xor_asm.go file that has appropriate build tags.

Change-Id: I555d44b2fd83f260a4855d83cacb9e101d689bc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/639856
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>

Author: 4a6f656c

Diff for: src/crypto/internal/fips140/subtle/xor_arm64.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.
 
-//go:build !purego
+//go:build (amd64 || arm64 || loong64 || ppc64 || ppc64le) && !purego
 
 package subtle