x/build: add LUCI linux-riscv64 builder #80880

Name: linux-riscv64-jorropo-sg2042

It is a 64 harts machine with GORISCV64=rva20u64 support which is the minimum we support but aren't testing on native rva20u64 hardware.
The build time is identical to the 4 harts P550 however the P550 got a multicore ratio of 3.2x , while this has a ratio of 13x on a 64 core machine so lots of room to grow as we improve the compiler's parallelism.
I didn't tried ./all.bash but I suspect it runs faster since it should be very multithreadable compared to compiling ssa and linking.

We could also easily run 4 builders from the same box and should get ~4x the performance.
The single core performance is abysmal, if you can use all 64 cores it's ok.

Architecture wise it got lots of Thead vendor extensions but I don't think we'll want to use any of them (almost all of them have been implemented 1:1* in official riscv extension).
Sadly the vector support is disabled (it supports a vendor extension similar to RVV 0.7.1 so we wouldn't use it anyhow) because you can decide the addressing mode from userland, including physical addressing mode (which I may say is pretty fast compared to virtual memory).

It should have 128GB of ram but I can only use 96GB otherwise it doesn't boot for some reason (power spike apparently ?).

It runs a fork of debian 13 (RevyOS).

It has fast-missaligned support so would be nice as an extension.

*except the encoding thus it can't use the official ones.