internal/platform,cmd/dist: export the list of supported platforms

internal/platform,cmd/dist: export the list of supported platforms

Also switch internal/platform to commit the generated code instead of
regenerating it in cmd/dist. Nothing in the generated code depends on
the target configuration, and committing the source file makes it
more amenable to searching and indexing (particularly on
https://cs.opensource.google/go/go).

For #60939.

Change-Id: I9133dfd5129b3c4d7457267589dfac5e7ecbef65
Reviewed-on: https://go-review.googlesource.com/c/go/+/505175
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>