runtime: make fatal failures unrecoverable

Match the Go runtime by terminating for deadlocks, stack overflows,
runtime and GC invariants, invalid lock operations, and platform
initialization failures instead of routing them through panic/recover.

Keep language-level runtime errors and unsupported user operations
recoverable. Add crash coverage that verifies fatal errors bypass deferred
recover calls.

src/runtime/runtime_tinygoriscv_qemu.go

@@ -117,7 +117,7 @@ func handleInterrupt() {
 				sleepCheckpoint.Jump()
 			}
 		default:
-			runtimePanic("unknown interrupt")
+			runtimeFatal("unknown interrupt")
 			abort()
 		}
 	} else {
@@ -200,7 +200,7 @@ func coreStackTop(core uint32) uintptr {
 	case 3:
 		return uintptr(unsafe.Pointer(&stack3TopSymbol))
 	default:
-		runtimePanic("unexpected core")
+		runtimeFatal("unexpected core")
 		return 0
 	}
 }
@@ -367,7 +367,7 @@ func (l *spinLock) Lock() {
 func (l *spinLock) Unlock() {
 	// Safety check: the spinlock should have been locked.
 	if schedulerAsserts && l.Uint32.Load() != 1 {
-		runtimePanic("unlock of unlocked spinlock")
+		runtimeFatal("unlock of unlocked spinlock")
 	}
 
 	// Unlock the lock. Simply write 0, because we already know it is locked.
@@ -417,7 +417,7 @@ func schedulerUnlockAndWait() {
 	// We can do this check since this is not baremetal: there won't be any
 	// external interrupts that might unblock a goroutine.
 	if sleepingHarts == (1<<numCPU)-1 {
-		runtimePanic("all cores are sleeping - deadlock!")
+		runtimeFatal("all cores are sleeping - deadlock!")
 	}
 
 	// Need to disable interrupts while saving the checkpoint, otherwise if the