This cuts 6 seconds off all.bash with the new go command.
Not a ton, but also an easy 6 seconds to grab.

The -tags=use_go_run in the misc/cgo tests is just some
go command flag that will make run.go use go run,
but without making everything look stale.
(Those tests have relative imports,
so go tool compile+link is not enough.)

Change-Id: I43bf4bb661d3adde2b2d4aad5e8f64b97bc69ba9
Reviewed-on: https://go-review.googlesource.com/73994Reviewed-by: Ian Lance Taylor <iant@golang.org>