__X32_SYSCALL_BIT not checked

man seccomp:
> The arch field is not unique for all calling conventions. The x86-64 ABI and the x32 ABI both use AUDIT_ARCH_X86_64 as arch, and they run on the same processors. Instead, the mask __X32_SYSCALL_BIT is used on the system call number to tell the two ABIs apart.

> This means that in order to create a seccomp-based blacklist for system calls performed through the x86-64 ABI, it is necessary to not only check that arch equals AUDIT_ARCH_X86_64, but also to explicitly reject all system calls that contain __X32_SYSCALL_BIT in nr.

Apparently, `__X32_SYSCALL_BIT` is not checked. Meaning that if a policy is compiled for x86_64, blacklists certain syscalls but the default action is `ALLOW`, a 32-bit caller will bypass the blacklist.

```
$ echo "DENY{SYSCALL[10]}DEFAULT ALLOW" | ./tools/dump_policy_bpf/dump_policy_bpf
BPF program with 7 instructions
0: A := architecture
1: if A != 0xc000003e goto 5
2: A := syscall number
3: if A < 0xa goto 6
4: if A >= 0xb goto 6
5: KILL
6: ALLOW
```