It's not clear this really belongs anywhere at all,
but this is a better place for it than package os.
This way package os can avoid importing "C".

Fixes #10455.

Change-Id: Ibe321a93bf26f478951c3a067d75e22f3d967eb7
Reviewed-on: https://go-review.googlesource.com/12574Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>