nimble/porting: Make os_mbuf/os_mpool work on 64-bit.

[jimmo]

We're using NimBLE in MicroPython and do much of our testing on (64-bit) Linux. @dpgeorge

Similar to #96 (@turon). Please see commit messages for details.

@KKopyscinski / @sjanc should I try and get this fixed in mynewt-core instead?

[jimmo]

Yes, lets fix it in core and then resync here. In general I think we should simply start using uintptr where needed (although mynewt was/is 32bit os only).

Thanks -- I'm not too familiar with mynewt (outside of NimBLE). Do you want me to follow this up or is this best left up to the maintainers?

Could you explain why is this needed? (AFAIR initially it was like that to allow multiple initialization while executing tests)

I'm not too fussed either way, but I noticed that nothing in NimBLE calls os_mempool_module_init and there's no declaration for it in any header files. So this change seems like the safest way to prevent implementors missing it.