alpha-linux-user: Fix signal handling

Proper signal numbers were not defined, and EXCP_INTERRUPT
was unhandled, leading to all sorts of subtle confusion.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

linux-user/main.c

@@ -2921,6 +2921,9 @@ void cpu_loop(CPUAlphaState *env)
         case EXCP_STQ_C:
             do_store_exclusive(env, env->error_code, trapnr - EXCP_STL_C);
             break;
+        case EXCP_INTERRUPT:
+            /* Just indicate that signals should be handled asap.  */
+            break;
         default:
             printf ("Unhandled trap: 0x%x\n", trapnr);
             cpu_dump_state(env, stderr, fprintf, 0);