Two of these error messages are already dead code: cmd/compile.main
and cmd/link.main already switch on $GOARCH, ensuring it must be a
prefix of the sys.Arch.Family.

The error message about uncompiled Go source files can be just be
simplified: anyone who's manually constructing Go object file archives
probably knows what tool to use to compile Go source files.

Change-Id: Ia4a67c0a1d1158379c127c91e909226d3367f3c2
Reviewed-on: https://go-review.googlesource.com/21626
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>