[aarch64] Converting from floating-point overflow issue

Currently the conversions from floating points (FPTOSI, FPTOUI) will only work correctly when there is an overflow (i.e. the source is larger or smaller than the largest/smallest values in the destination) for 32-bit and 64-bit, and 128-bit (I should probably test this). For other sizes it will just truncate the result of the 32-bit/64-bit operation, which will not produce the correct value if the floating point overflows the smaller type.