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


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