tccgen: gen_assign_cast(): cannot cast struct to scalar

tccgen: gen_assign_cast(): cannot cast struct to scalar

The case below previously was causing an assertion failure
in the target specific generator.

It probably is not incorrect not to allow this even if
gcc does.

    struct S { long b; };

    void f(struct S *x)
    {
        struct S y[1] = { *x };
    }