Blackfin: split optimization settings more

We need to place icache flush funcs into L1 inst sram to work around a
hardware anomaly.  But this currently breaks SMP support as the L1 inst
sram is per-core and cannot be called directly.  So in preparation for
making that work, split the two options.

Further, split out the SMP depend so that we can allow some for SMP.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>