shared/seccomp: do not use ifdef guards around textual syscall names

It is possible that we will be running with an upgraded libseccomp, in which
case libseccomp might know the syscall name, even if the number is not known at
the time when systemd is being compiled. The guard only serves to break such
upgrades, by requiring that we also recompile systemd.

For s390-specific syscalls, use a define to exclude them, so that that we don't
try to filter them on other arches.

(cherry picked from commit 6da432fd)