libstdc++: Use copy_file_range for filesystem::copy_file

copy_file_range is a recent-ish syscall for copying files. It is similar
to sendfile but allows filesystem-specific optimizations. Common are:
Reflinks: BTRFS, XFS, ZFS (does not implement the syscall yet)
Server-side copy: NFS, SMB, Ceph

If copy_file_range is not available for the given files, fall back to
sendfile / userspace copy.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (_GLIBCXX_USE_COPY_FILE_RANGE): Define.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* src/filesystem/ops-common.h (copy_file_copy_file_range):
	Define new function.
	(do_copy_file): Use it.

Signed-off-by: Jannik Glückert <jannik.glueckert@gmail.com>