seccomp: move arch_prctl to @default

It was reported as used by the linker:

> [It is] called in the setup of ld-linux-x86-64.so.2 from _dl_sysdep_start.
> My local call stack (with LTO):
>
> #0 init_cpu_features.constprop.0 (/usr/lib64/ld-linux-x86-64.so.2)
> #1 _dl_sysdep_start (/usr/lib64/ld-linux-x86-64.so.2)
> #2 _dl_start (/usr/lib64/ld-linux-x86-64.so.2)
> #3 _start (/usr/lib64/ld-linux-x86-64.so.2)
>
> Looking through the source, I think it's this (links for glibc 2.34):
> - First dl_platform_init calls _dl_x86_init_cpu_features, a wrapper for init_cpu_features.
> - Then init_cpu_features calls get_cet_status.
> - At last, get_cet_status invokes arch_prctl.

Fixes #22033.

(cherry picked from commit 5f02870a)