Fix Arm __ASSUME_COPY_FILE_RANGE (bug 23915) [committed]

[Florian Weimer]

* Joseph Myers:

> 2018-11-23  Joseph Myers  <joseph@codesourcery.com>
>
> 	[BZ #23915]
> 	* sysdeps/unix/sysv/linux/arm/kernel-features.h
> 	[__LINUX_KERNEL_VERSION < 0x040700] (__ASSUME_COPY_FILE_RANGE):
> 	Undefine.

So I feel kind of bad about this—I looked for such issues after the
first problems cropped up and just didn't seem them in the kernel source
tree.

Assuming that I'm going to implement a few system call wrappers in the
future, how can I do a better job here?

Thanks,
Florian