[kernel] Add vmar alignment flags

This change introduces a set of flags of the form

ZX_VM_ALIGN_<number> that can be combined with other ZX_VM
flags to control the |address| alignement of
- zx_vmar_allocate(...., &address);
- zx_vmar_map(..., &address);

TEST: unit tests added

ZX-1173 #done

Change-Id: I7a6920ec16d1b89a21c36850e99fe336f3c9270d