cmd/toolstash: use 'go tool dist list' to get platforms
cmd/toolstash: use 'go tool dist list' to get platforms

This fixes several issues:

* The ls sed hackery generates GOARCH values mipsx and mipsxle,
  instead of mips, mips64, mipsle, and mips64le.
* It removes the dependence on the 9 command.
* It marginally increases readability.

Change-Id: Ie46abc289c2c9923eed693dcf1194f4fd3ea9bd2
Reviewed-on: https://go-review.googlesource.com/38347
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,golang/tools