This implements rseq for amd64, arm, arm64, ppc32, ppc64,
s390x and x86 linux as ENOSYS (without warning).
glibc will start using rseq to accelerate sched_getcpu, if
available. This would cause a warning from valgrind every
time a new thread is started.
Real rseq (restartable sequences) support is pretty hard, so
for now just explicitly return ENOSYS (just like we do for clone3).
https://sourceware.org/pipermail/libc-alpha/2021-December/133656.html