[sw-sysemu] Fix rvtimer write_mtimecmp immediate interrupt raise

When mtimecmp is written to a value already in the past (mtime >= mtimecmp)
and the interrupt was not previously asserted, raise the timer interrupt
immediately. This mirrors hardware behavior where the comparator fires as
soon as the condition is met.

If the interrupt was already asserted (MIP bit already set), no action is
needed — the hart already has a pending interrupt.

Without this fix, tickless kernels that write mtimecmp to a nearly-passed
value can deadlock: the internal interrupt flag is set but the hart MIP bit
is never updated, so the ISR never fires and no subsequent write_mtimecmp
call is made.

Author: AFOliveira

sw-sysemu/devices/rvtimer.h

@@ -57,6 +57,13 @@ struct RVTimer
                     agent.chip->clear_machine_timer_interrupt(shire);
                 }
             }
+        } else if (!had_interrupt && interrupt) {
+            // raise immediately if mtimecmp is written in the past.
+            for (unsigned shire = 0; shire < EMU_NUM_SHIRES; ++shire) {
+                if ((interrupt_shire_mask >> shire) & 1) {
+                    agent.chip->raise_machine_timer_interrupt(shire);
+                }
+            }
         }
     }