Fix non-asm builds (#1)

Add build tag checks to assembly. Add missing function to fallback.

Author: klauspost

crc64_amd64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a license that can be
 // found in the LICENSE file.
 
+//go:build !noasm && !appengine && !gccgo
+
 #include "textflag.h"
 
 TEXT ·updateAsm(SB), $0-40

crc64_arm64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a license that can be
 // found in the LICENSE file.
 
+//go:build !noasm && !appengine && !gccgo
+
 #include "textflag.h"
 
 TEXT ·updateAsm(SB), $0-40

crc64_other.go

@@ -7,3 +7,5 @@
 package crc64nvme
 
 var hasAsm = false
+
+func updateAsm(crc uint64, p []byte) (checksum uint64) { panic("should not be reached") }