runtime: fix newosproc darwin+arm/arm64

Missed conversion of newosproc for the parts of darwin that
weren't affected by my previous change.

Update #25181

Change-Id: I81a2935e192b6d0df358c59b7e785eb03c504c23
Reviewed-on: https://go-review.googlesource.com/110123


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>