[libc] Add utility functions for warp-level scan and reduction (#84866)

Summary:
The GPU uses a SIMT execution model. That means that each value actually
belongs to a group of 32 or 64 other lanes executing next to it. These
platforms offer some intrinsic fuctions to actually take elements from
neighboring lanes. With these we can do parallel scans or reductions.
These functions do not have an immediate user, but will be used in the
allocator interface that is in-progress and are generally good to have.
This patch is a precommit for these new utilitly functions.