syscall: implement wasip1 Fcntl

syscall: implement wasip1 Fcntl

CL 494915 broke non-blocking I/O on wasip1 for files
opened with os.NewFile. This is fixed by providing an
implementation of fcntl(F_GETFL) for wasip1.

Change-Id: I78979076b95495fd4b94814552e5f5b043270cd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/498195
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>