Adding helper function for ARM "dsb st"

[Boyer, Andrew]

I see that cnxk has added inline assembly for the ARM "dsb st" barrier. We, too, would like to add a use of this instruction. Can a helper function be added to the atomics headers?

> drivers/ml/cnxk/cn10k_ml_dev.h:#define dsb_st ({ asm volatile("dsb st" : : : "memory"); })

What would the helper be named, rte_wsb()? I suppose it would be a NOP on all other platforms.

Maybe we could add something for "dmb st" at the same time and get rid of that assembly, too.

Thanks,
Andrew