fix(riscv64): implement fixed-parameter and return-saving calling convention

[gounthar]

Summary

Master restored the LLVM backend with a riscv64 target, but SubstrateRISCV64RegisterConfig.getCallingConvention still throws "Fixed parameter assignments and return saving are not yet supported on this platform" for custom-ABI calls. The first foreign call during image generation (throwNewNullPointerException) trips it, so native-image cannot get past "Compiling methods" on riscv64.

This implements the fixed-parameter-assignment and return-saving path for riscv64, mirroring the existing AArch64 register config: the return-buffer placeholder parameter is parked in x28 (t3) where AArch64 uses r8, and custom-ABI parameters are assigned to their fixed register/stack locations. The standard-ABI path is unchanged in behaviour, only moved under the !type.customABI() branch.

Related Issues

Part of reviving the riscv64 LLVM backend, discussed in #13351.

Testing

Built substratevm for linux-riscv64 on a Banana Pi F3 (SpaceMiT K1) and ran native-image on a HelloWorld with --tool:llvm-backend --no-fallback. Before this change the build aborted in getCallingConvention at the first foreign call; after it, the build proceeds through all method compilation. The image does not yet link on riscv64 due to a separate, independent issue (the in-process object reader still uses the LLVM 13 bytedeco preset against LLVM 20.1.4 objects, discussed in #13351).

Documentation

No documentation changes needed.

Contributor Checklist

• I have read the contribution guide.
• I have the right to contribute the submitted material under the project terms.
• I have updated tests and documentation where appropriate.
• If I used a coding assistant, I remain responsible for the entire contribution and have reviewed it accordingly.