RISCV64 support

[JuniorJPDJ]

Would be cool to see manylinux images for riscv64 (musllinux cannot be easily supported as alpine stable still doesn't have support for riscv). This architecture is rising fast now ;D

[mayeut]

This will depend on RHEL support for risk-v (or a derivative like AlmaLinux/RockyLinux).
Until then, nothing will happen as using ubuntu or a debian derivative as a base image is most likely a no-go here (c.f. #1012)

[JuniorJPDJ]

Ubuntu 20.04 is the first officially supporting RISC-V distro anyway and it has GCC9 which is new enough. Why not just support RISC-V on Ubuntu 20.04 starting from manylinux_2_31? Old distro releases will anyway not add new CPU architecture support, so we are covered.

[mayeut]

Even if we wanted to add a base image using ubuntu 20.04 which I'm still not sure wouldn't be seen as a regression by the community (unless adding only a RISC-V image), the question of CI would remain. qemu is a no go here as it is way to slow to build images (we were often hitting GHA time limit on ppc64le when travis-ci was broken on that architecture).

[JuniorJPDJ]

I was thinking about adding this images for only RISC-V. QEMU is a problem here then.

[yzewei]

I was thinking about adding this images for only RISC-V. QEMU is a problem here then.

easy
use buildx...

[mayeut]

well buildx still uses qemu, that's what was used for ppc64le on GHA