libstdc++: Also use sendfile for big files

We were previously only using sendfile for files smaller than 2GB, as
sendfile needs to be called repeatedly for files bigger than that.

Some quick numbers, copying a 16GB file, average of 10 repetitions:
    old:
        real: 13.4s
        user: 0.14s
        sys : 7.43s
    new:
        real: 8.90s
        user: 0.00s
        sys : 3.68s

libstdc++-v3/ChangeLog:

	* acinclude.m4 (_GLIBCXX_HAVE_LSEEK): Define.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* src/filesystem/ops-common.h (copy_file_sendfile): Define new
	function for sendfile logic. Loop to support large files. Skip
	zero-length files.
	(do_copy_file): Use it.

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