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

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 <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>