Seccomp filter: deal with arch_prctl(ARCH_CET_STATUS, ...).

Seccomp filter: deal with arch_prctl(ARCH_CET_STATUS, ...).

The dynamic loader of GNU libc 2.28 uses this system call to
initialize CPU information, see
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/cpu-features.c;hb=glibc-2.28#l28.
Simulating an older kernel by returning EINVAL should be the most
harmless rule here.

The ARCH_CET_STATUS symbol isn't yet exposed by the kernel headers;
see the FIXME at the top of
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/include/asm/prctl.h;hb=glibc-2.28.

* lib-src/seccomp-filter.c (ARCH_CET_STATUS): Define if not
already present.  Inline the value because there doesn't seem to
be a header file exporting this constant yet.
(main): Make ARCH_CET_STATUS subfunction of arch_prctl return EINVAL.