[$] Fixing the TCMalloc regression with RSEQ operations

The restartable sequences feature is one ofthe stranger corners of the kernel's user-space interface; it provides away for user space to carry out simple lockless operations and be informedif it is preempted over the course of an operation (and must, thus,restart). Work merged in the 6.19 release to improve the performance of restartablesequences broke the TCMalloc allocator,which was relying on an undocumented (and unintended) kernel behavior.Now, Olivier Dion is proposing anaddition to the restartable-sequences API that will bring TCMalloc backinto the fold; it does not make the restartable-sequences API any lessstrange, though.