[MIPS] Sibyte: Build RTC support as an object

 Build the SWARM platform library is as an object rather than an archive
so that files which only contain symbols used by initcalls and do not
provide any symbols that would pull them from an archive still work.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>