[libc] Implement efficient 'malloc' on the GPU (#140156)

Summary:
This is the big patch that implements an efficient device-side `malloc`
on the GPU. This is the first pass and many improvements will be made
later.

The scheme revolves around using a global reference counted pointer to
hand out access to a dynamically created and destroyed slab interface.
The slab is simply a large bitfield with one bit for each slab. All
allocations are the same size in a slab, so different sized allocations
are done through different slabs.

Allocation is thus searching for or creating a slab for the desired
slab, reserving space, and then searching for a free bit. Freeing is
clearing the bit and then releasing the space.

This interface allows memory to dynamically grow and shrink. Future
patches will have different modes to allow fast first-time-use as well
as a non-RPC version.

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