cmd/dist: use purego tag when building the bootstrap binaries
This is in addition to the current math_big_pure_go tag.
Using purego ensures that we can build the cmd binaries with gccgo.
For #53662
Change-Id: Ib82f8bf10659b5f94935f2b427ae8b2da875cd3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/415934 Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>