x64: Rearrange cnode_cap structure to improve fastpath speed

x64: Always set the high bits of certain pointers in the fastpath

seL4 is always in the top of memory, so the high bits of pointers are always 1.
The autogenerated unpacking code doesn't know that, however, so will try to
conditionally sign extend (in 64-bit mode), which wastes cycles in the fast
path. Instead, we can do the unpacking ourselves and explicitly set the high
bits.

x64: Efficiently pack objects for fastpath

Improves the packing of structures used in the fastpath

x64: Fastpath

SELFOUR-317: rename async endpoint to notification object, and other
fallout.

Refactor ARM and ia32 fastpath to use a common code base