cmd/dist: do not rewrite files that already have the correct content

cmd/dist: do not rewrite files that already have the correct content

In particular, this avoids moving the mtime on runtime/zversion.go
forward unless the file is out of date. In turn, this makes cross compiles
that run dist multiple times coexist nicely.

(It's no longer necessary to run dist multiple times to set up cross compiles,
but people still might, and it's easy to fix regardless.)

Fixes #4749.

Change-Id: Id430525f168f106bc4b821ca74b2ca498a748f14
Reviewed-on: https://go-review.googlesource.com/9152
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>