wasm: use precise GC for WebAssembly (including WASI)

With a few small modifications, all the problems with `-gc=precise` in
WebAssembly seem to have been fixed.

I didn't do any performance measurements, but this is supposed to
improve GC performance.

src/internal/task/task.go

@@ -30,3 +30,6 @@ type Task struct {
 // the given function and falls back to the default stack size. It is replaced
 // with a load from a special section just before codegen.
 func getGoroutineStackSize(fn uintptr) uintptr
+
+//go:linkname runtime_alloc runtime.alloc
+func runtime_alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer