[clang][WebAssembly] Imply -fno-threadsafe-static when threading is disabled

When we don't enable atomics we completely disabled threading in
which case there is no point in generating thread safe code for
static initialization.

This should always be safe because, in WebAssembly, it is not
possible to link object compiled without the atomics feature into a
mutli-threaded program.

See https://github.com/emscripten-core/emscripten/pull/16152

Differential Revision: https://reviews.llvm.org/D118571