syscall: avoid "64"-postfixed libSystem syscalls on iOS

The stat(2) man page contain this comment about the 64-bit versions
of the system file functions:

"Platforms that were released after these updates only have the
newer variants available to them.  These platforms have the macro
_DARWIN_FEATURE_ONLY_64_BIT_INODE defined."

It turns out that on iOS the _DARWIN_FEATURE_ONLY_64_BIT_INODE is
defined and that even though the "64"-postfixed versions are
accessible they are deemed private. Apps that refer to private
API are not admissible on App Store, and after the Go runtime
started using libSystem instead of direct syscalls, the App Store
submission checks reject apps built with Go tip.

The fix is simple: use the non-postfixed versions on iOS.

getdirentries(2) is not changed; it is not available at all on iOS
and needs replacement.

Updates #28984

Change-Id: Icb8d44e271456acaa1913ba486fcf5b569722fa9
Reviewed-on: https://go-review.googlesource.com/c/151938


Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>