[Bpf] ISA: BPF_CALL | BPF_X

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clang generates BPF code with opcode 0x8d (BPF_CALL | BPF_X, which it calls
"callx"),
when compiling with -O0 or -O1.  Of course -O2 is recommended, but if anyone
later
defines opcode 0x8d for anything other than what clang means by it, it could
cause
problems.

On the BPF_MSH thread at
https://mailarchive.ietf.org/arch/msg/bpf/ogmS9qFhdBCxC4VrOWL7nzjSiXU/
Alexei wrote regarding BPF_ABS and BPF_IND:
> DW never existed in classic bpf, so abs/ind never had DW flavor.
> If some assembler/compiler decided to "support" them it's on them.
> The standard must not list such things as deprecated. They never existed.

Technically BPF_CALL | BPF_X never existed either, so can be omitted from
the IANA registry.  But given the widespread deployment of clang's
use, and the WG charter statement:
> The BPF working group is initially tasked with documenting the existing
> state of the BPF ecosystem
I could see a potential argument to list it as reserved or something.

Today, the document doesn't reserve it, so it's open for future use for any
purpose.  I just wanted to verify that the WG is ok with not listing it
in the IANA registry, given the above information.

Dave