cmd/dist: build bootstrap without GOEXPERIMENT

Currently, dist attempts to build the bootstrap with the GOEXPERIMENT
set in the environment. However, the logic is incomplete and notably
requires a hack to enable the appropriate build tags for
GOEXPERIMENT=regabi. Without this hack, the build becomes skewed
between a compiler that uses regabi and a runtime that doesn't when
building toolchain2.

We could try to improve the GOEXPERIMENT processing in cmd/dist, but
it will always chase cmd/internal/objabi and it's quite difficult to
share the logic with objabi because of the constraints on building
cmd/dist.

Instead, we switch to building go_bootstrap without any GOEXPERIMENT
and only start using GOEXPERIMENT once we have a working, modern
cmd/go (which has all the GOEXPERIMENT logic in it). We also build
toolchain1 without any GOEXPERIMENT set, in case the bootstrap
toolchain is recent enough to understand build-time GOEXPERIMENT
settings.

As part of this, we make GOEXPERIMENT=none mean "no experiments". This
is necessary since, now that we support setting GOEXPERIMENT at build
time, we need an explicit way to say "ignore all baked-in experiments".

For #40724.

Change-Id: I115399579b766a7a8b2f352f7e5efea5305666cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/302050


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>