os: set File.appendMode in NewFile if file was opened with O_APPEND

os: set File.appendMode in NewFile if file was opened with O_APPEND

To allow skipping the use of the copy_file_range syscall on Linux which
isn't supported for destination files opened with O_APPEND, see comment
in (*File).readFrom and
https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS

Fixes #60181

Change-Id: Ie0b0050faab16858412928a3d1f96442619581eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/494915
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>