runtime: document all possible values for GOOS and GOARCH

The updated list is taken from "src/go/build/syslist.go".

Reason: one should not do web search to know the possible values of GOOS
and GOARCH.  The search result point to stackoverflow page which
reference the above source and documentation on installation page [1].
It should available offline (as in local godoc), as part of package
documentation.

[1] https://golang.org/doc/install/source#environment

Change-Id: I736804b8ef4dc11e0260fa862999212ab3f7b3fd
Reviewed-on: https://go-review.googlesource.com/129935Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/extern.go

@@ -238,6 +238,7 @@ func Version() string {
 
 // GOOS is the running program's operating system target:
 // one of darwin, freebsd, linux, and so on.
+// To view possible combinations of GOOS and GOARCH, run "go tool dist list".
 const GOOS string = sys.GOOS
 
 // GOARCH is the running program's architecture target: