cmake toolchain file for wasm32-wasi-pthread

[yamt]

currently wasi-sdk.cmake has set(triple wasm32-wasi).
there needs a way to use set(triple wasm32-wasi-pthread) for threaded apps.

also, set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread") is required to pass the cmake compiler test (CMakeTestCCompiler.cmake) for wasm32-wasi-pthread.

a straightforward solution is to have a separate cmake toolchain file, say wasi-sdk-pthread.cmake for threaded apps.

[abrown]

I think is closed by #274.