Reformat "Platform Variants", especially to add amd64, ppc64le, riscv64

[tianon]

This changes the table to be a line per ISA instead of per variant; adds x86-64, POWER8+, and RISC-V; and notes the appropriate "Go analog" in each case (because these values mirror Go's choices, like our use of GOOS and GOARCH).

Of particular note, GOARM64 and GORISCV64 are not fully implemented or released in Go yet, but they are accepted proposals with in-progress implementations or already merged to main:

• GOARM64: all: add GOARM64=v8.1 and so on golang/go#60905
• GORISCV64: all: add GORISCV64 environment variable golang/go#61476 (comment)

Closes #852

[tianon]

(or be similar to)

This part makes me nuts, but I felt like I had to somehow address that GOARM does not use a v prefix, GOARM64 will not (at least initially) support v8 (without a minor, like v8.0), we don't do extra things like softfloat or sse2 in this field, etc SOMEHOW in this wording.

[xnox]

Hm, i would be more explicit in tags that are supported. One can reference go constants as valid ones, but specify them explicitly.

There will always be variants that are not yet specified or relevant for go, but do make sense to be explicit for container images that contain code that was written in other languages. For example, whilst GOARM64 is already defined, it doesn't actually do anything different in go - but it does for gcc for example.

[tianon]

The table below does include explicit examples, though 😅

[tianon]

Putting these links here because I keep wanting them and having to find them again (will maybe update this over time with more interesting/useful links):

• https://ubuntu.com/blog/optimising-ubuntu-performance-on-amd64-architecture
• https://developers.redhat.com/blog/2021/01/05/building-red-hat-enterprise-linux-9-for-the-x86-64-v2-microarchitecture-level
• https://gitlab.archlinux.org/archlinux/rfcs/-/blob/master/rfcs/0002-march.rst
• https://fedoraproject.org/wiki/Changes/x86-64_micro-architecture_update

[tianon]

In case it's helpful to someone else, I'll note here that I've copied the current debian:trixie-slim to tianon/test:variants, filtered the list to those with an appropriate variant value that could be set, and added the appropriate and explicit variant values in both the config and index so folks can test runtime behavior:

• https://oci.dag.dev/?image=tianon/test:variants
• https://oci.dag.dev/?image=tianon/test@sha256:91edd9ae2beeaae8858813bb0f30df15d32a6a815d853246172cb1691e93baea (tianon/test@sha256:91edd9ae2beeaae8858813bb0f30df15d32a6a815d853246172cb1691e93baea)

Each architecture also exists as a single-architecture tagged image manifest so runtime edge case behavior can be more easily fully tested:

• tianon/test:variant-amd64
• tianon/test:variant-arm64v8
• tianon/test:variant-arm32v7
• tianon/test:variant-arm32v5
• tianon/test:variant-ppc64le
• tianon/test:variant-riscv64

Edit: updated arm64v8 image to use v8.0 explicitly as a better testing target / more likely to break things

Edit 2: containerd/platforms#8 (comment) 👀