add riscv64im-unknown-openvm-elf tier 3 target

[Tuanlinh12312]

Add riscv64im-unknown-openvm-elf as a tier 3 Rust target for the OpenVM zero-knowledge virtual machine (RV64IM ISA). The target includes std support via the OpenVM platform abstraction layer.

[jonathanpwang]

Should we change the target to some openvm branch so we can merge it?
also any way to get CI to not break, or it has too many rust org stuff?

[shuklaayush]

Should we change the target to some openvm branch so we can merge it?

can't we just close the pr after review and maintain the changes in the branch?

also any way to get CI to not break, or it has too many rust org stuff?

probably not, but the release workflow did work and we have a tarball
https://github.com/openvm-org/rust/releases/tag/openvm-nightly-2026-01-18

[jonathanpwang]

Should we change the target to some openvm branch so we can merge it?

can't we just close the pr after review and maintain the changes in the branch?

also any way to get CI to not break, or it has too many rust org stuff?

probably not, but the release workflow did work and we have a tarball https://github.com/openvm-org/rust/releases/tag/openvm-nightly-2026-01-18

We can, but given that this is a target we want to maintain, I'd just say we rename the branch to something more official