RISCV: Add some definitions and vararg supports

[Ast-x64]

This is based on RISCV Calling Convention and hints in comments (e.g.
elf/elf.h in glibc).

[dlang-bot]

Thanks for your pull request and interest in making D better, @Ast-x64! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3846"

[ljmf00]

Looks good. Anyone with experience in the RISC-V ABI to double-check this?

[ibuclaw]

Looks good. Anyone with experience in the RISC-V ABI to double-check this?

va_list is indeed a void* on RISC-V. Can't say much about the other parts of varargs and dwarfeh, other than seem reasonable. I assume this is for the benefit of LDC? (GDC has __builtin_eh_return_data_regno and __builtin_va_arg intrinsics exposed from the backend)

[Ast-x64]

I assume this is for the benefit of LDC?

Yes I'm trying to port LDC to riscv64gc platform. My current draft version shows, printf (which uses vararg to pass parameters) is working properly under my tests.