[$] User-space spinlocks with help from rseq()

Back in May, Andre Almeida presented somework toward the creation of user-space spinlocks using adaptivespinning. At that time, the work was stalled because there is, in Linux,currently no way to quickly determine whether a given thread is actuallyexecuting on a CPU. Some progress has since been made on that front; atthe 2023Open Source Summit Europe, Almeida returned to discuss how thatdifficulty might be overcome.