C-SKY: mark lr as undefined to stop unwinding

Mark the lr register as undefined at the start of execution, so unwind
will stop at this frame. run-backtrace-*.sh from elfutils testsuite will
fail without this patch.

	* sysdeps/csky/abiv2/start.S: Mark lr as undefined.
	* sysdeps/unix/sysv/linux/csky/abiv2/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/csky/abiv2/setcontext.S: Likewise.