[Blackfin] arch: a rather old performance improvement for the signal handling code

This is a rather old performance improvement for the signal handling
code, which was originally only committed on the 2007R1 branch as a
workaround for what we suspected to be a hardware bug.

There's no point in constructing a sigreturn stub on the stack and
flushing caches; we can just make signal handlers return to a known
location in the fixed code area.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>