cmd/dist: set GOARM=7 for windows/arm

GOARM=6 executables fail to launch on windows/arm, so set this to ARMv7
like we do for Android.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.

Change-Id: Ifa13685e7ab6edd367f3dfec10296e376319dbd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/291629


Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>