Future CLs will be changing the provenance of these functions. Move the
declarations to the individual OS files now so that future CLs can
change only 1 OS at a time rather than changing all at once.

For #51087

Change-Id: I5e1bca71e670263d8c0faa586c1b6b4de1a114b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/388474


Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>