cmd/dist: reproducibility fixes

cmd/dist: reproducibility fixes

Fix a few lingering reproducibility problems.

- Do not set CC during go install std if it is unset,
  so that the automatic disabling of cgo in cmd/go can run.

- Since CC is not necessary, remove code insisting on it.

- Use a fixed quoting algorithm instead of %q from the
  bootstrap toolchain, which can differ from release to release.

- Remove go_bootstrap tool successfully on Windows.

For #24904.

Change-Id: I5c29ba6a8592e93bfab37f123b69f55c02f12ce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/475377
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>