default to single-threaded for WebAssembly

When targeting WebAssembly, we default to building a single-threaded build
as threads are still experimental. The user however can enable a multi-
threaded build by specifying '-fno-single-threaded'. It's a compile-error
to enable this flag, but not also enable shared-memory.

lib/std/Thread.zig

@@ -919,6 +919,10 @@ const WasiThreadImpl = struct {
 
     /// Bootstrap procedure, called by the host environment after thread creation.
     export fn wasi_thread_start(tid: i32, arg: *Instance) void {
+        if (builtin.single_threaded) {
+            // ensure function is not analyzed in single-threaded mode
+            return;
+        }
         __set_stack_pointer(arg.thread.memory.ptr + arg.stack_offset);
         __wasm_init_tls(arg.thread.memory.ptr + arg.tls_offset);
         @atomicStore(u32, &WasiThreadImpl.tls_thread_id, @intCast(tid), .SeqCst);