build/env: add cross compile toolchain for armhf

Moved arm to armel, if anyone wants that in the future.

Change-Id: I57db5c8dff12c02cd2ceb4bd657d5fa63040985c
Reviewed-on: https://go-review.googlesource.com/29684
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
6 files changed