m68k/coldfire: flush cache when creating the signal stack frame

When the signal stack frame is created, it must be flushed in order to
make sure the cache fetches the correct data.
Without cache flush the icache might pick up old cached data from an older
signal stack frame if the signal is raised again very fast.
In case of copyback the data cache muist be pushed first, but is untested.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>