RFR (M): 8188764: Obsolete AssumeMP and then remove all support for non-MP builds

[Doerr, Martin]

Hi David,

> It is still used for spin-loop and related optimisations. Badly tuned 
> spinloops on uniprocessors could be a significant performance issue. 
> Keeping the is_MP check on a MP system is no hindrance as its just part 
> of the spin-loop.

So this means you're actually fixing this.
os::is_MP is always true in the current implementation (due to "AssumeMP ||").
Thanks for explaining.

Shared code part looks fine.

Thanks,
Martin