The Go 1.6 release notes say we'll remove the “-X name value” form
(in favor of the “-X name=value” form) in Go 1.7.
Do that.

Also establish the doc/go1.7.txt file.

Change-Id: Ie4565a6bc5dbcf155181754d8d92bfbb23c75338
Reviewed-on: https://go-review.googlesource.com/19614Reviewed-by: Ian Lance Taylor <iant@golang.org>