[dev.typeparams] internal/goarch,internal/goos: rename Goos and Goarch constants

Lots of constants in these packages start with Goarch and Goos, which is
redundant. Instead, add the "Is" prefix to make the constant clearer,
and to differentiate from the arch family constants.

Change-Id: Id92c1d3e911296a72949f8c9d649f142e7dc9d17
Reviewed-on: https://go-review.googlesource.com/c/go/+/328343


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>