go/build: introduce go1.7 build tag

This is needed now for subrepos to be able to conditionally use
API symbols found only after Go 1.6.

Change-Id: Ie7d9301332aa1739b585d93f8025424ae72a2430
Reviewed-on: https://go-review.googlesource.com/20344Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/build.go

@@ -270,7 +270,7 @@ func defaultContext() Context {
 	// in all releases >= Go 1.x. Code that requires Go 1.x or later should
 	// say "+build go1.x", and code that should only be built before Go 1.x
 	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
-	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6"}
+	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7"}
 
 	switch os.Getenv("CGO_ENABLED") {
 	case "1":

src/go/build/doc.go

@@ -103,6 +103,7 @@
 //	- "go1.4", from Go version 1.4 onward
 //	- "go1.5", from Go version 1.5 onward
 //	- "go1.6", from Go version 1.6 onward
+//	- "go1.7", from Go version 1.7 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // If a file's name, after stripping the extension and a possible _test suffix,