cmd/go: adjust BuildInfo.Settings

Make Settings more closely align with command-line flags
and environment variables.

- Change command-line flags to begin with -

- Change syntax of build lines to use Key=Value instead of Key<tab>Value.

- Change CGO_ENABLED to 0/1, matching environment variable,
  instead of false/true.

- Add GOOS and GOARCH.
  These are technically redundant, in that they can be extracted
  from the binary in other ways most of the time, but not always:
  GOOS=ios and GOOS=darwin may produce binaries that are
  difficult to tell apart. In any case, it's a lot easier to have them
  directly in the settings list than derive them from other parts
  of the binary.

- Add GOEXPERIMENT.
  These could be inferred from the tags list, but the experiments
  are being removed from the tags list.

- Change the tags list to match the -tags command-line argument.

- Add msan and race, echoing the -msan and -race arguments
  (always 'true' when present, omitted when false).

- Add GO$GOARCH when set.

Change-Id: Icb59ef4faa5c22407eadd94147b7e53cf4344ce6
Reviewed-on: https://go-review.googlesource.com/c/go/+/358539


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>