I was previously setting GOARM=arm5 (due to confusion with previously
seeing buildall.sh's temporary of "arm5" as a GOARCH and
misremembernig), but GOARM=arm5 was acting like GOARM=5 only on
accident. See https://go-review.googlesource.com/#/c/10023/

Instead, fail if GOARM is not a known value.

Change-Id: I9ba4fd7268df233d40b09f0431f37cd85a049847
Reviewed-on: https://go-review.googlesource.com/10024Reviewed-by: Minux Ma <minux@golang.org>