runtime: Plan 9, 32-bit: fix build by updating call to open()
runtime: Plan 9, 32-bit: fix build by updating call to open()

With the global redefinition of runtime·open by CL 7543043,
we need to provide a third argument and remove the cast
to the string.

Fixes build on 386 version of Plan 9.

R=khr, rsc, rminnich, ality
CC=golang-dev
https://codereview.appspot.com/7644047

Committer: Ron Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go