Deprecate x86 support.

There's lots of reasons why this is a good idea:

• Weaker ASLR.
• Seccomp related headaches.
  • I wanted to use gosecco to do runtime seccomp rule generation, but it only supports amd64.
  • libseccomp2 shipped on Debian stable generates wrong code on amd64, so the current workaround of pre-generating compiled bpf at build time using libseccomp2 from backports precludes runtime code generation, degrading sandbox effectiveness across all platforms.
  • x86 systems can't effectively filter out arguments to a lot of socket related system calls because of socketcall().
• Supporting hardware I don't have, running software I don't use, to ultimately obtain results that are empirically worse than the other supported platform is a poor use of development time.
• Tails gave up on supporting 32 bit userland (https://labs.riseup.net/code/issues/8183).