syscall: fix fsync for read-only files on aix

syscall: fix fsync for read-only files on aix

AIX fsync syscall doesn't work on read-only files. Using fsync_range
instead allows syscall.Fsync to work on any files.

Fixes #41372

Change-Id: I66d33e847875496af53da60828c1bddf6c2b76b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/254657
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>