docs: add riscv64 native build instructions

[gounthar]

Summary

Add documentation for building WASI SDK from source on native riscv64 Linux hardware.

The build completes with zero source patches — only cmake configuration flags are needed:

1. -DCMAKE_C_COMPILER_WORKS=ON to skip compiler test (Stage 1 Clang only targets wasm32)
2. -DWASI_SDK_TARGETS="wasm32-wasi;wasm32-wasip1" to skip WASIP2 (wit-bindgen/wasm-tools/wkg don't ship riscv64 binaries)
3. Post-install symlink for the Clang resource directory

Tested on Banana Pi F3 (SpacemiT K1, rv64gc, 8 cores, 16 GB RAM, Debian Trixie) with WASI SDK v30. Full self-hosted loop verified: C → Clang (native rv64) → .wasm → iwasm (native rv64).

Ref: #607

What this enables

Self-hosted WebAssembly development on RISC-V hardware without needing an x86 or ARM host for compilation.

Test plan

• Built WASI SDK v30 on Banana Pi F3 (rv64gc) following these instructions
• Compiled hello-world.c to .wasm using the native riscv64 toolchain
• Ran the .wasm with iwasm on the same board

[alexcrichton]

Thanks for this! I'm not sure that this documentation is all that much different from the README.md though? The extra parts here I think boil down to (a) wasip2/wasip3 can't be built due to host tools not being available and (b) the clang-resource-dir bit which can be fixed by configuring with -DWASI_SDK_INSTALL_TO_CLANG_RESOURCE_DIR=ON.

Given that I think it might be reasonable to add a note in the README about disabling wasip2/wasip3 for non-allow-listed architectures (or perhaps better yet, codify that in CMake). Another possible alternative would be implementing/documenting downloading the wasi-sysroot from this repository and plopping it into a toolchain. There's no need to build the sysroot on all platforms, for example.