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>