net: do not use sendfile on iOS

Apparently, on the iOS builder sendfile causes a SIGSYS signal
(instead of returning ENOSYS). Disabling it for now so we can
make progress on iOS. We can revisit if sendfile is actually
broken on iOS and whether it is beneficial.

Updates #49616.

Change-Id: I3883fad0ce35e3f0aa352301eb499a1afa0225a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/368054
Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Changkun Ou <mail@changkun.de>
Reviewed-by: Roland Shoemaker <roland@golang.org>

Author: cherrymui

src/net/sendfile_stub.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || (js && wasm) || netbsd || openbsd
+//go:build aix || (js && wasm) || netbsd || openbsd || ios
 
 package net
 

src/net/sendfile_unix_alt.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build darwin || dragonfly || freebsd || solaris
+//go:build (darwin && !ios) || dragonfly || freebsd || solaris
 
 package net