Ignore purego build constraint

`purego` is a proposed[1] de-facto build constraint to
denote building _only_ Go code.

We must ignore this constraint to make sure that files
that toggle on `purego` are added to the Bazel target
`srcs` and let the compiler decide if they should be
compiled or not.

[1]: golang/go#23172

language/go/build_constraints.go

@@ -313,7 +313,7 @@ func matchAuto(tokens []string) (*buildTags, error) {
 // build constraints for a file and will instead defer to the compiler at compile
 // time.
 func isIgnoredTag(tag string) bool {
-	if tag == "cgo" || tag == "race" || tag == "msan" {
+	if tag == "cgo" || tag == "purego" || tag == "race" || tag == "msan" {
 		return true
 	}
 	if len(tag) < 5 || !strings.HasPrefix(tag, "go") {