Add riscv64 build, make target list more explicit

[threexc]

maturin supports riscv64 cross-compile, so add that to the build matrix. However, first make things more explicit about which rustc targets to use for each architecture.

From previous explorations of riscv64 builds, I've found that just specifying riscv64 as a target doesn't work, because the rustc target is actually riscv64gc-unknown-linux-gnu. We could simply add riscv64gc to the matrix, but that won't fit the patterns used for final packages, so take the time to add a platform list with separate target and arch fields.

Note that this is being done on behalf of the RISE Project to improve Python ecosystem support on riscv64 platforms.

Please also note that we have a previous PR that is still open: #107

[meta-cla]

Hi @threexc!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[ezyang]

need CLA