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Introduce AddPerCpu and StorePerCpu over an rseq-sharded per-CPU array.
On the x86-64 Linux fast path the update is committed by a hand-rolled
rseq critical section (non-atomic, migration-safe): addq for the 8-byte
accumulate, movq / movdqu for the 8- or 16-byte store. The kernel
restarts the sequence on preemption or migration, and only one thread
runs on a CPU at a time, so no atomic or lock is needed. Off the fast
path (other arches, no kernel rseq) the operation falls back to an
atomic on the slot indexed by sched_getcpu().
A naturally-aligned 8-byte store is single-copy atomic on x86-64, so it
is never observed torn; the 16-byte store may be, which is acceptable for
a last-writer-wins gauge.
commit_hash:6250f6e9e35cf3895ebafe0b534ec12cca50b03b
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