Support for RISC-V 64-bit GNU/Linux

[msizanoen1]

Add support for RISC-V 64-bit GNU/Linux.

Follow up to rust-lang/rust#66661.

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[gnzlbg]

Please add a build job for this target here to make sure that this at least compiles: https://github.com/rust-lang/libc/blob/master/ci/build.sh#L224

You might want to consider taking over #1416 as a follow up (no need to try to do any of that here).

[gnzlbg]

@bors: r+

[gnzlbg]

@bors: r+

[bors]

📌 Commit 112b7f9 has been approved by gnzlbg

[msizanoen1]

The target specification is not in the latest nightly, only master - even when using rustup update --force.
It looks like we will have to wait for next nightly.

[gnzlbg]

@bors: r+

[bors]

📌 Commit a06978b has been approved by gnzlbg

[bors]

⌛ Testing commit a06978b with merge 5130285...

[bors]

☀️ Test successful - checks-cirrus-freebsd-10, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, status-azure
Approved by: gnzlbg
Pushing 5130285 to master...