runtime: remove go prefix from a few routines

They are no longer needed now that C is gone.

goatoi -> atoi
gofuncname/funcname -> funcname/cfuncname
goroundupsize -> already existing roundupsize

Change-Id: I278bc33d279e1fdc5e8a2a04e961c4c1573b28c7
Reviewed-on: https://go-review.googlesource.com/2154
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>