kern/xcall: optimize and improve documentation

Don't uselessly align to the CPU cache line size. Remove the useless array
of preallocated cross-calls, and allocate from the caller stack instead.
Since that array was the only object protected by disabling preemption,
leave preemption enabled accordingly.

Document locking keys, and describe memory ordering guarantees and enforcing.
Add TODO entry about asynchronous cross-call support.

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