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

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