runtime: rename atoi to _atoi on Plan 9

Following change 2154, the goatoi function
was renamed atoi.

However, this definition conflicts with the
atoi function defined in the Plan 9 runtime,
which takes a []byte instead of a string.

This change fixes the build on Plan 9.

Change-Id: Ia0f7ca2f965bd5e3cce3177bba9c806f64db05eb
Reviewed-on: https://go-review.googlesource.com/2165
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>