irq: Introduce and use CPU_INTERRUPT_SSTEP_MASK.

This mask contains all of the bits that should be ignored while single
stepping in the debugger.  The mask contains 2 bits that are not currently
cleared, but are also never set.  The bits are included in the mask for
consistency in handling of the CPU_INTERRUPT_TGT_EXT_N bits.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/cpu-exec.c b/cpu-exec.c
index 395cd8c..5b42b25 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -360,10 +360,7 @@
                 if (unlikely(interrupt_request)) {
                     if (unlikely(env->singlestep_enabled & SSTEP_NOIRQ)) {
                         /* Mask out external interrupts for this step. */
-                        interrupt_request &= ~(CPU_INTERRUPT_HARD |
-                                               CPU_INTERRUPT_FIQ |
-                                               CPU_INTERRUPT_SMI |
-                                               CPU_INTERRUPT_NMI);
+                        interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
                     }
                     if (interrupt_request & CPU_INTERRUPT_DEBUG) {
                         env->interrupt_request &= ~CPU_INTERRUPT_DEBUG;