seccomp: allow shmat to be a separate syscall on architectures which use a multiplexer

After
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0d6040d46817,
those syscalls have their separate numbers and we can block them.
But glibc might still use the old ones. So let's just do a best-effort
block and not assume anything about how effective it is.