kernel: Add C++ main() support

The C++ standard requires the main() function to have the return type
of 'int' and does not allow the main() to be defined with the 'void'
return type. Moreover, GCC goes as far as to emit a hard error when the
'::main()' has the return type of `void`.

This commit introduces an option to instruct the Zephyr kernel to call
the 'int main(void)' instead of the 'void main(void)' in case a Zephyr
application defines main() in a C++ source file.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>