Implement linux rseq syscall as ENOSYS

Implement linux rseq syscall as ENOSYS

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