The code can be made portable with a few simple macros (which is why they are implemented they way they are in GCC), though the optimizations will not be.

From the Linux kernel:

    struct boot_params boot_params __attribute__((aligned(16)));

    #if !__GNUC__
    #define __attribute__(x)
    #endif

I think that's a better way to do it than ASM (which totally hoses your portability).