[release-branch.r57] Make.cmd: create TARGDIR if necessary

««« CL 4437089 / b2670a39da7c
Make.cmd: create TARGDIR if necessary

Fixes #1771.

R=adg, rsc1
CC=golang-dev
https://golang.org/cl/4437089
»»»

R=adg
CC=golang-dev
https://golang.org/cl/4454054