cmd/go: put computed GOROOT in built binaries

As of CL 42533, cmd/go will recompute its GOROOT based on the
location of its own executable. This CL plumbs that computed GOROOT
into every binary it builds using the linker -X flag. This
means binaries built with a moved cmd/go will report the GOROOT
they were built in from runtime.GOROOT().

Fixes #21313

Change-Id: I6c2c559f40f2a0c867ab60cf47c6dbc73ae5e28a
Reviewed-on: https://go-review.googlesource.com/61310


Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>