docs: add riscv64 native build instructions

Document how to build WASI SDK from source on native riscv64 Linux
hardware. The build requires zero source patches — only cmake flags
for skipping compiler tests and limiting to WASIP1 targets.

Tested on Banana Pi F3 (SpacemiT K1, rv64gc, 16 GB RAM, Debian Trixie)
with WASI SDK v30. Stage 1 (LLVM) takes ~8-10 hours, Stage 2 (sysroot)
takes ~15-20 minutes.

Ref: https://github.com/WebAssembly/wasi-sdk/issues/607

docs/riscv64-build.md

@@ -0,0 +1,115 @@
+# Building WASI SDK on riscv64
+
+WASI SDK can be built from source on native riscv64 Linux hardware. The build
+completes with zero source patches — only cmake configuration flags are needed.
+
+## Prerequisites
+
+- **Hardware**: Any rv64gc board with 16+ GB RAM (tested on Banana Pi F3,
+  SpacemiT K1)
+- **OS**: Debian Trixie or equivalent with riscv64 packages
+- **Toolchain**: GCC 14+, cmake 3.26+, ninja-build, python3, git
+
+Install build dependencies (Debian/Ubuntu):
+
+```bash
+sudo apt-get install -y build-essential cmake ninja-build python3 git ccache
+```
+
+## Build process
+
+WASI SDK builds in two stages:
+
+1. **Stage 1 (Toolchain)**: Builds LLVM, Clang, and LLD targeting WebAssembly.
+   This is a full LLVM build and takes 8-10 hours on an 8-core rv64gc board.
+2. **Stage 2 (Sysroot)**: Uses the Stage 1 compiler to build wasi-libc. Takes
+   ~15-20 minutes.
+
+### Stage 1: Build the toolchain
+
+```bash
+git clone --recursive https://github.com/WebAssembly/wasi-sdk.git
+cd wasi-sdk
+
+cmake -G Ninja -B build/toolchain -S . \
+  -DWASI_SDK_BUILD_TOOLCHAIN=ON \
+  -DCMAKE_BUILD_TYPE=MinSizeRel \
+  -DCMAKE_INSTALL_PREFIX=$(pwd)/build/install
+
+ninja -C build/toolchain install
+```
+
+### Stage 2: Build the sysroot
+
+The Stage 1 compiler only targets `wasm32`, so cmake's compiler test will fail.
+Use `-DCMAKE_C_COMPILER_WORKS=ON` to skip it (the CI build already does this).
+
+WASIP2 targets require `wit-bindgen`, `wasm-tools`, and `wkg`, which are Rust
+tools that don't ship riscv64 binaries. Limit targets to WASIP1:
+
+```bash
+cmake -G Ninja -B build/sysroot -S . \
+  -DCMAKE_TOOLCHAIN_FILE=$(pwd)/build/install/share/cmake/wasi-sdk.cmake \
+  -DCMAKE_C_COMPILER_WORKS=ON \
+  -DCMAKE_CXX_COMPILER_WORKS=ON \
+  -DWASI_SDK_TARGETS="wasm32-wasi;wasm32-wasip1" \
+  -DCMAKE_INSTALL_PREFIX=$(pwd)/build/install
+
+ninja -C build/sysroot install
+```
+
+### Fix resource directory path
+
+After installation, create a symlink for the Clang resource directory so the
+compiler can find its runtime libraries:
+
+```bash
+CLANG_VERSION=$(build/install/bin/clang --version | grep -oP '\d+\.\d+\.\d+')
+CLANG_MAJOR=${CLANG_VERSION%%.*}
+ln -sf $(pwd)/build/install/lib/clang/${CLANG_MAJOR}/lib \
+       $(pwd)/build/install/lib/clang-resource-dir/lib
+```
+
+## Verification
+
+```bash
+# Compile a test program
+cat > hello.c << 'EOF'
+#include <stdio.h>
+int main() { printf("Hello from WASI SDK on riscv64!\n"); return 0; }
+EOF
+
+build/install/bin/clang --target=wasm32-wasi --sysroot=build/install/share/wasi-sysroot \
+  -o hello.wasm hello.c
+
+file hello.wasm
+# hello.wasm: WebAssembly (wasm) binary module version 0x1 (MVP)
+```
+
+If you have `iwasm` (from WAMR) installed:
+
+```bash
+iwasm hello.wasm
+# Hello from WASI SDK on riscv64!
+```
+
+## Build statistics
+
+Measured on Banana Pi F3 (SpacemiT K1, 8x rv64gc @ 1.6 GHz, 16 GB RAM):
+
+| Metric | Value |
+|--------|-------|
+| Stage 1 (LLVM+Clang+LLD) | ~8-10 hours |
+| Stage 2 (wasi-libc sysroot) | ~15-20 minutes |
+| Installed SDK size | 244 MB |
+| hello.wasm output | 105 KB |
+| Source patches needed | 0 |
+
+## Known limitations
+
+- **WASIP2 not supported**: `wit-bindgen`, `wasm-tools`, and `wkg` do not ship
+  riscv64 binaries. Only `wasm32-wasi` and `wasm32-wasip1` targets can be built.
+- **No CI integration yet**: GitHub Actions does not provide riscv64 runners.
+  QEMU-based builds are possible but would take many hours.
+- **`wasm-component-ld` not built**: This Rust tool does not cross-compile
+  easily to riscv64. It is only needed for WASIP2 component linking.