wasi,cc: fix naming and add stubs for building

Rename include dir to match the convention:
  from `wasm32-wasi` to `wasm-wasi-musl`

Add building stubs which will be used to build and cache WASI
libc sysroot.

CMakeLists.txt

@@ -593,6 +593,7 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/src/translate_c/ast.zig"
     "${CMAKE_SOURCE_DIR}/src/type.zig"
     "${CMAKE_SOURCE_DIR}/src/value.zig"
+    "${CMAKE_SOURCE_DIR}/src/wasi_libc.zig"
     "${CMAKE_SOURCE_DIR}/src/windows_sdk.zig"
     "${CMAKE_SOURCE_DIR}/src/Zir.zig"
     "${CMAKE_SOURCE_DIR}/src/Sema.zig"