cmd/dist: set GOARM=7 for windows/arm
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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Trust: Jason A. Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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