arch: posix: Declare _posix_zephyr_main with int return type

This commit updates the `_posix_zephyr_main` declaration to use the
return type of `int` instead of `void` when `CONFIG_CPP_MAIN=y` (i.e.
C++-compliant main() support is enabled) so that Zephyr applications
defining their main() in a C++ source file can make use of the proper
main() definition of `int main(void)` as required by the C++ standard.

Note that the forward declaration of `_posix_zephyr_main` is required
if and only if the main() is defined in a C++ source file (i.e. when
`CONFIG_CPP_MAIN=y`).

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