web-wasi: Bump to WASI-SDK 17

The binary linux SDK in version 17 requires a newer glibc than we have
available, so we have to use our own patched clang. This follows the
approach in:

  https://github.com/WebAssembly/wasi-sdk/pull/271

web-wasi/clang-wasi-sysroot.sh

@@ -1,3 +1,3 @@
 #!/usr/bin/env sh
 
-exec ${WASI_SDK_PATH}/bin/clang --sysroot=${WASI_SYSROOT} "$@"
+exec clang-${LLVM_VERSION} --target=wasm32-wasi --sysroot=${WASI_SYSROOT} "$@"