all: go 1.17 (#197)

Bump CI Go versions to 1.16 and 1.17.
Update build tags with `go:build` equivalents.
Upgrade asmfmt tool for new `go:build` support.

Updates #183

examples/add/README.md

@@ -6,6 +6,7 @@ The [code generator](asm.go) is as follows:
 
 [embedmd]:# (asm.go)
 ```go
+//go:build ignore
 // +build ignore
 
 package main

examples/add/asm.go

@@ -1,3 +1,4 @@
+//go:build ignore
 // +build ignore
 
 package main