cmd/buildlet: halt the machine for Sierra VMs on MacStadium
cmd/buildlet: halt the machine for Sierra VMs on MacStadium

macOS Sierra's network stack gets wedged after a few runs of all.bash.
Instead, halt the machine after a build is complete. cmd/makemac will make
a new one for the next build.

Change-Id: Icb1b21c045376cff9958a0d3b4cfaf784c9948d0
Reviewed-on: https://go-review.googlesource.com/37465
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build