cmd/dist: don't use "uname -v" to recognize GOHOSTARCH

cmd/dist: don't use "uname -v" to recognize GOHOSTARCH

We can use processor architecture or hardware platform as part of
hostname and it leads to misconfiguration of GOHOSARCH.

For example,

$ uname -m -v
FreeBSD 10.1-RELEASE-p5 #0: Tue Jan 27 08:52:50 UTC 2015 root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC i386

Change-Id: I499efd98338beff6a27c03f03273331ecb6fd698
Reviewed-on: https://go-review.googlesource.com/4944
Reviewed-by: Minux Ma <minux@golang.org>