web-wasi-threads: initial addition

This is a different toolchain following how wasi-sdk defines the
platform triple and how builds are configured -- everything should be
built with the flags defined in the toolchain file.

We build from web-wasi:latest, and that is built first.

common/common.webassembly

@@ -47,6 +47,7 @@ RUN mkdir /wasi-runtimes
 ENV WASMTIME_HOME /wasi-runtimes/wasmtime
 RUN mkdir ${WASMTIME_HOME} && curl https://wasmtime.dev/install.sh -sSf | bash
 COPY imagefiles/wasmtime-pwd.sh ${WASMTIME_HOME}/bin/
+COPY imagefiles/wasmtime-pwd-threads.sh ${WASMTIME_HOME}/bin/
 ENV PATH "$WASMTIME_HOME/bin:$PATH"
 
 # For wasmer