Fix a compilation issue on ARM (missing definition of EM_ARM).
Update the road-map for v3.1, v3.2, and v3.3.
Fix uint8_t overflow in the seccomp multi-arch support.

With seccomp enabled, the support for x86 on x86_64 was broken because
of this integer overflow.  Previously, the BPF program was:

  line  OP   JT   JF   K
  =================================
  0000: 0x20 0x00 0x00 0x00000004   ld  $data[<arch_offset>]
  0001: 0x15 0x00 0x00 0xc000003e   jeq <arch_number> true:0002 false:0002
  0002: ...

Now, the BPF program is:

  line  OP   JT   JF   K
  =================================
  0000: 0x20 0x00 0x00 0x00000004   ld  $data[<arch_offset>]
  0001: 0x15 0x01 0x00 0xc000003e   jeq <arch_number> true:0003 false:0002
  0002: 0x05 0x00 0x00 0x00000100   jmp 0259
  0003: ...