Use the arm_sync_file_range to implement SyncFileRange.

This is an alias for sync_file_range2 syscall which is already used to
implement SyncFileRange on ppc64x. These have the order of the flags and
offset arguments reversed in order to fit the syscall arguments within 6
registers.

For more information see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=edd5cd4a9424f22b0fa08bef5e299d41befd5622

This makes TestSyncFileRange build and pass on linux/arm.

Change-Id: I1316328ebcf550a3fb70f9ce5ebf54d8bcaa932c
Reviewed-on: https://go-review.googlesource.com/c/153939
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>