Add riscv64gc-unknown-linux-gnu image

[JohnTitor]

r? @ghost

[JohnTitor]

Seems ctest needs to add that target:

--- stderr
thread 'main' panicked at 'unknown arch/pointer width: riscv64gc-unknown-linux-gnu', /cargo/registry/src/github.com-1ecc6299db9ec823/ctest-0.2.22/src/lib.rs:1080:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Hey @gnzlbg, would you mind inviting me to ctest repo as a collaborator? Or if you have time to review, I'm happy to submit a PR!

[bors]

☔ The latest upstream changes (presumably #1695) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnTitor]

Okay, CI is now green.
cc @msizanoen1 and @andreas-schwab, could you take a look if you have time? Looks like you're familiar with RISC-V than me :)

[andreas-schwab]

ipc_perm now uses mode_t everywhere.

[JohnTitor]

Skipping this because it was introduced in the newest version of glibc and I'd like to wait for a while. On riscv, isn't it newer change?

[andreas-schwab]

I have been building rust for riscv64 sucessfully for some time now, see https://build.opensuse.org/package/show/openSUSE:Factory:RISCV/rust and https://build.opensuse.org/package/show/home:Andreas_Schwab:riscv:rust/rust.

[andreas-schwab]

I think this is only needed because rust is still using llvm9, with llvm10 the riscv64gc-unknown-linux-gnu target should use the proper defaults now.

[JohnTitor]

This PR makes some fields private that shouldn't be public so I believe the build shouldn't be broken unless someone depends on that accidentally.

[JohnTitor]

So, I'd like to merge this since it seems not to affect current behavior. Thanks all for reviewing!