kernel: 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

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

kernel/main_weak.c

@@ -21,16 +21,10 @@
 
 #include <kernel_internal.h>
 
-#ifdef CONFIG_CPP_MAIN
 int __weak main(void)
-#else
-void __weak main(void)
-#endif
 {
 	/* NOP default main() if the application does not provide one. */
 	arch_nop();
 
-#ifdef CONFIG_CPP_MAIN
 	return 0;
-#endif
 }