[MIPS] Fix __fls for non-MIPS32/MIPS64 cpus

Only MIPS32 and MIPS64 CPUs implement clz/dclz. Therefore don't export
__ilog2() for non MIPS32/MIPS64 cpus and use generic __fls bitop code for
these cpus.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>