Each architecture's Rconv function is only used inside its
respective package, so it does not need to be exported.

Change-Id: Ifbd629964d7a9edd66501d7cdf4750621d66d646
Reviewed-on: https://go-review.googlesource.com/39110
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>