bpf, docs: Expand set of initial conformance groups

bpf, docs: Expand set of initial conformance groups

This patch attempts to update the ISA specification according
to the latest mailing list discussion about conformance groups,
in a way that is intended to be consistent with IANA registry
processes and IETF 118 WG meeting discussion.

It does the following:
* Split basic into base32 and base64 for 32-bit vs 64-bit base
  instructions
* Split division/multiplication/modulo instructions out of base groups
* Split atomic instructions out of base groups

There may be additional changes as discussion continues,
but there seems to be consensus on the principles above.

v1->v2: fixed typo pointed out by David Vernet

v2->v3: Moved multiplication to same groups as division/modulo

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240202221110.3872-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>