Support riscv64 platform

[eschnett]

No description provided.

[eschnett]

Question: Is this

    # We have experimental support for some platforms, allow easily including them
    if experimental
        append!(standard_platforms, [
            Platform("riscv64", "linux"),
            Platform("riscv64", "linux"; libc="musl"),
        ])
    end

reasonable? That is, do we tag this platform as "experimental", and do we include a musl build?

For context: Musl is much easier to install than Glibc, and "historically" speaking, musl was working for me before glibc did. We should acknowledge this.

[giordano]

I don't think we're going to use "experimental" anymore, in hindsight it was a bit cumbersome.

About musl, we could perhaps have it, but in practice Julia has lots of troubles running on musl (we can't even compile it for i686, and other architectures like x86_64 crash continuously) that I'm not so sure it's worth the effort adding it.

[giordano]

I can confirm simple executables built on this PR with

$ julia +1.7 -e 'using BinaryBuilderBase; BinaryBuilderBase.runshell(Platform("riscv64", "linux"))'
sandbox:${WORKSPACE} # echo -e '#include <stdio.h>\nint main(void) { printf("Hello world!\\n"); return 0; }' | gcc -x c - -o hello-gcc
sandbox:${WORKSPACE} # echo -e '#include <stdio.h>\nint main(void) { printf("Hello world!\\n"); return 0; }' | clang -x c - -o hello-clang

both work successfully on RISC-V:

$ ./hello-clang 
Hello world!
$ ./hello-gcc
Hello world!

🥳

[giordano]

I think this is basically good to go from my point of view as soon as JuliaPackaging/BinaryBuilder.jl#1355 (comment) is worked out (I don't have strong opinions about that).

[eschnett]

We haven't found any evidence for page sizes other than 4 kB, so let's go with the current state.