libseccomp has as dependency at least aarch64. This can be extended to also be available when BR2_arm=y. Libseccomp supports this now (and also in 2.2.0 which is currently referenced in buildroot).
Fixed by https://git.buildroot.org/buildroot/commit/?id=bd2c2329c534dd45d2b269c26fb630aaa761b1d4. Thanks!