This fixes a bug where double arguments on a 32-bit architecture would not
be passed correctly because they only had 4 bytes of storage (not 8). It
also fixes a compiler warning/error in return_ffi_value on certian
architectures: array subscript 'double[0]' is partly outside array bounds
of 'ffi_arg[1]' {aka 'long unsigned int[1]'}.
Fixes issue #7064.
Signed-off-by: Damien George <damien@micropython.org>