syscall: use asynchronous operations on js/wasm

This commit makes syscall on js/wasm use the asynchronous variants
of functions in Node.js' fs module. This enables concurrency
and allows the API of the fs module to be implemented with an
alternative backend that only supports asynchronous operations.

Updates #26051.

Change-Id: Ibe1dcc988469fc11c3b8d8d49de439c12ddaafce
Reviewed-on: https://go-review.googlesource.com/c/137236


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>