runtime: document all possible values for GOOS and GOARCH

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/129935
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>