[cmake] make WASM_TOOLCHAIN mandatory (for a while)

CMakeModules/wasm.cmake

-find_package(Wasm REQUIRED)
+find_package(Wasm)
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(WASM_TOOLCHAIN DEFAULT_MSG WASM_FOUND)
 
+if (NOT WASM_TOOLCHAIN_FOUND)
+    return()
+endif()
+
 macro(compile_wast)
   #read arguments include ones that we don't since arguments get forwared "as is" and we don't want to threat unknown argument names as values
   cmake_parse_arguments(ARG "NOWARNINGS" "TARGET;DESTINATION_FOLDER" "SOURCE_FILES;INCLUDE_FOLDERS;SYSTEM_INCLUDE_FOLDERS;LIBRARIES" ${ARGN})