Add a decent set of syscall tests for rseq(2). These are a bit awkward because
of issues with library integration. libc may register rseq on thread start
(including before main on the initial thread), precluding much testing. Thus we
run tests in a libc-free subprocess.
Support for rseq(2) in gVisor will come in a later commit.
PiperOrigin-RevId: 284595994
498595d543