proto: support purego build tag

The proposal in golang/go#23172 was accepted. The "purego" build tag
is intended to be a community agreed upon soft-signal to indicate the
forbidden use of unsafe, assembly, or cgo.

A change in the future will remove special-casing the appengine and js tags
once the related toolchains support purego (possibly after some bake-in period).

proto/pointer_reflect.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build appengine js
+// +build purego appengine js
 
 // This file contains an implementation of proto field accesses using package reflect.
 // It is slower than the code in pointer_unsafe.go but it avoids package unsafe and can

proto/pointer_unsafe.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build !appengine,!js
+// +build !purego,!appengine,!js
 
 // This file contains the implementation of the proto field accesses using package unsafe.