This fixes errors like these ones:

modffi.c: In function 'return_ffi_value':
modffi.c:143:29: error: cast to pointer from integer of different size
[-Werror=int-to-pointer-cast]
             const char *s = (const char *)val;
                             ^
modffi.c:162:20: error: cast to pointer from integer of different size
[-Werror=int-to-pointer-cast]
             return (mp_obj_t)val;
                    ^
modffi.c: In function 'ffifunc_call':
modffi.c:358:25: error: cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
             values[i] = (ffi_arg)a;
                         ^
modffi.c:373:25: error: cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
             values[i] = (ffi_arg)s;
                         ^
modffi.c:381:25: error: cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
             values[i] = (ffi_arg)bufinfo.buf;
                         ^
modffi.c:384:25: error: cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]
             values[i] = (ffi_arg)p->func;
                         ^

These errors can be highlighted when building micropython from MIPS64
n32 because ffi_arg is 64-bit wide and the pointers on MIPS64 n32 are
32-bit wide, so it's trying to case an integer to a pointer (or
vice-versa) of a different size. We should cast first the pointer (or the
integer) to a pointer sized integer (intptr_t) to fix that problem.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>