samples, tests, boards: Switch main return type from void to int

As both C and C++ standards require applications running under an OS to
return 'int', adapt that for Zephyr to align with those standard. This also
eliminates errors when building with clang when not using -ffreestanding,
and reduces the need for compiler flags to silence warnings for both clang
and gcc.

Most of these changes were automated using coccinelle with the following
script:

@@
@@
- void
+ int
main(...) {
	...
-	return;
+	return 0;
	...
}

Approximately 40 files had to be edited by hand as coccinelle was unable to
fix them.

Signed-off-by: Keith Packard <keithp@keithp.com>

samples/boards/stm32/power_mgmt/blinky/src/main.c

@@ -15,7 +15,7 @@
 static const struct gpio_dt_spec led =
 	GPIO_DT_SPEC_GET(DT_ALIAS(led0), gpios);
 
-void main(void)
+int main(void)
 {
 	bool led_is_on = true;
 
@@ -37,4 +37,5 @@ void main(void)
 		}
 		led_is_on = !led_is_on;
 	}
+	return 0;
 }

samples/boards/stm32/power_mgmt/serial_wakeup/src/main.c

@@ -11,14 +11,14 @@
 #include <zephyr/sys/printk.h>
 #include <zephyr/sys/__assert.h>
 
-void main(void)
+int main(void)
 {
 	const struct device *const dev =
 		DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
 
 	if (!device_is_ready(dev)) {
 		printk("Console device not ready");
-		return;
+		return 0;
 	}
 
 #if CONFIG_PM_DEVICE
@@ -51,4 +51,5 @@ void main(void)
 	}
 #endif
 
+	return 0;
 }

samples/boards/stm32/power_mgmt/stm32wb_ble/src/main.c

@@ -108,7 +108,7 @@ static void bt_ready(int err)
 	printk("Beacon stopped\n");
 }
 
-void main(void)
+int main(void)
 {
 	int err;
 
@@ -148,4 +148,5 @@ void main(void)
 
 	printk("Shutdown\n");
 	pm_state_force(0u, &(struct pm_state_info){PM_STATE_SOFT_OFF, 0, 0});
+	return 0;
 }