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: https://github.com/golang/go/issues/60905
• GORISCV64: https://github.com/golang/go/issues/61476#issuecomment-1791156741

Closes https://github.com/opencontainers/image-spec/issues/852

[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