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>

tests/bsim/bluetooth/host/gatt/caching/src/main.c

@@ -15,7 +15,8 @@ bst_test_install_t test_installers[] = {
 	NULL
 };
 
-void main(void)
+int main(void)
 {
 	bst_main();
+	return 0;
 }

tests/bsim/bluetooth/host/gatt/general/src/main.c

@@ -15,7 +15,8 @@ bst_test_install_t test_installers[] = {
 	NULL
 };
 
-void main(void)
+int main(void)
 {
 	bst_main();
+	return 0;
 }

tests/bsim/bluetooth/host/gatt/notify/src/main.c

@@ -15,7 +15,8 @@ bst_test_install_t test_installers[] = {
 	NULL
 };
 
-void main(void)
+int main(void)
 {
 	bst_main();
+	return 0;
 }

tests/bsim/bluetooth/host/gatt/notify_multiple/src/main.c

@@ -15,7 +15,8 @@ bst_test_install_t test_installers[] = {
 	NULL
 };
 
-void main(void)
+int main(void)
 {
 	bst_main();
+	return 0;
 }

tests/bsim/bluetooth/host/gatt/write/src/main.c

@@ -125,7 +125,8 @@ bst_test_install_t test_installers[] = {
 	NULL
 };
 
-void main(void)
+int main(void)
 {
 	bst_main();
+	return 0;
 }