switch compat_sys_sched_rr_get_interval to COMPAT_SYSCALL_DEFINE

... and make it unconditional - we want the sucker on all biarch
platforms, really.  All kinds of wrappers and private implementations
can go now; fortunately, they don't cause name conflicts, so we can
do that one first without any bisect hazards.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>