[POWERPC] Add __ucmpdi2 for 64-bit comparisons in 32-bit kernels

Some drivers (such as V4L2) have code that causes gcc to generate
calls to __ucmpdi2 when compiling for 32-bit powerpc, which results
in either a link-time error or a module that can't be loaded, as
we don't currently have a __ucmpdi2.  This adds one so these drivers
can be used.

Signed-off-by: Paul Mackerras <paulus@samba.org>