syscall: move Syscall declarations to OS files

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>