cmd/dist: default to GOARM=7 on all non-arm systems

If you run make.bash on an arm system without GOARM set,
we sniff the local system to find the maximum default GOARM
that will actually work on that system. That's fine, and we can
keep doing that.

But the story for cross-compiling is weirder.
If we build a windows/amd64 toolchain and then use it to
cross-compile linux/arm binaries, we get GOARM=7 binaries.
Do the same on a linux/amd64 system and you get GOARM=5 binaries.
This clearly makes no sense, and worse it makes the builds
non-reproducible in a subtle way.

This CL simplifies the logic and improves reproducibility by
defaulting to GOARM=7 any time we wouldn't sniff the local system.

On go.dev/dl we serve a linux-armv6l distribution with a default GOARM=6.
That is built by setting GOARM=6 during make.bash, so it is unaffected
by this CL and will continue to be GOARM=6.

For #24904.

Change-Id: I4331709876d5948fd33ec6e4a7b18b3cef12f240
Reviewed-on: https://go-review.googlesource.com/c/go/+/470695
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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