blackfin,kgdb: Do not put PC in gdb_regs into retx.

In blackfin, kgdb is running in delayed exception IRQ5 other than in
exception IRQ3 directly.  Register reti other than retx in pt_regs is
the kgdb return address. So, don't put PC in gdb_regs into retx.

CC: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sonic Zhang <sonic.adi@gmail.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>