Blackfin: fix single stepping over system calls

On Blackfin systems, the hardware single step exception triggers before
the system call exception, so we need to save this info to process it
later on.  Otherwise, single stepping in userspace misses a few insns
right after the system call.

This is based a bit on the SuperH code added in commit 4b505db9.

Reported-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>