kernel: Explicitly define the main prototype if C++

If the embedded application has a main(),
Zephyr requires it to have a signature with
C linkage (no C++ name mangling).
Otherwise Zephyr's init will not call it,
but will call the default weak stub.

But, when building with clang/llvm, when we
build calling the compiler with --frestanding
(for ex if CONFIG_MINIMAL_LIBC is set), the
compiler will mangle the main() symbol name.

This is not incorrect behavior from the compiler,
as, in principle, in a freestanding environment
main() does not have a special meaning.

gcc is still not mangling it when called with
--frestanding.

To avoid this issue, we define explicitly
the main linkage as extern C,
in a header the application will always include.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

include/zephyr/types.h

@@ -29,6 +29,11 @@ typedef union {
 	void            (*thepfunc)(void);
 } z_max_align_t;
 
+#ifdef __cplusplus
+/* Zephyr requires an int main(void) signature with C linkage for the application main if present */
+extern int main(void);
+#endif
+
 #ifdef __cplusplus
 }
 #endif