cmd/dist: detect gohostarch on netbsd/arm64 hosts

cmd/dist: detect gohostarch on netbsd/arm64 hosts

On netbsd/arm64 `uname -m` reports `evbarm` which is mapped to
gohostarch=arm. Fix this by checking for "aarch64" in `uname -p` output
to fix self-hosted build on netbsd/arm64.

Updates #30824

Change-Id: I0f0450ff35af0bec51aeec3b210143ba892216c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/257643
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>