Fix Arm __ASSUME_COPY_FILE_RANGE (bug 23915).

The generic kernel-features.h defines __ASSUME_COPY_FILE_RANGE for 4.5
and later kernels.  However, for 32-bit Arm binaries running on 64-bit
Arm kernels, the syscall was only wired up in the 4.7 kernel, although
the 32-bit Arm kernel had the syscall from 4.5 onwards.  This patch
corrects the Arm kernel-features.h to undefine the macro for
configured minimum kernel versions before 4.7.

Tested (compilation only) with a build-many-glibcs.py build for
arm-linux-gnueabi.

	[BZ #23915]
	* sysdeps/unix/sysv/linux/arm/kernel-features.h
	[__LINUX_KERNEL_VERSION < 0x040700] (__ASSUME_COPY_FILE_RANGE):
	Undefine.