[Blackfin] arch: fix gdb testing regression

When transferring to IRQ5 from an exception, save SYSCFG in memory across the
transfer and clear the trace bit.

When we get a single step exception, check whether we can safely clear the
trace bit in SYSCFG.  We can (and should) clear it after the first instruction
of the interrupt handler; the first insn saves SYSCFG to the stack in all
handlers.

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

include/asm-blackfin/entry.h

@@ -17,6 +17,11 @@
 #define	PF_DTRACE_OFF	1
 #define	PF_DTRACE_BIT	5
 
+/*
+ * NOTE!  The single-stepping code assumes that all interrupt handlers
+ * start by saving SYSCFG on the stack with their first instruction.
+ */
+
 /* This one is used for exceptions, emulation, and NMI.  It doesn't push
    RETI and doesn't do cli.  */
 #define SAVE_ALL_SYS		save_context_no_interrupts

include/asm-blackfin/mach-common/context.S

@@ -27,6 +27,11 @@
  * 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
+/*
+ * NOTE!  The single-stepping code assumes that all interrupt handlers
+ * start by saving SYSCFG on the stack with their first instruction.
+ */
+
 /*
  * Code to save processor context.
  *  We even save the register which are preserved by a function call