y2038: linux: Provide __futimens64 implementation

This patch provides new __futimens64 explicit 64 bit function for
setting access and modification time of file (by using its file descriptor).
Moreover, a 32 bit version - __futimens has been refactored to internally use
__futimens64.

The __futimens is now supposed to be used on systems still supporting
32 bit time (__TIMESIZE != 64) - hence the necessary conversions to 64 bit
struct __timespec64.
When pointer to struct __timespec64 is NULL - the file access and modification
time is set to the current one (by the kernel) and no conversions from struct
timespec to __timespec64 are performed.

The __futimens64 reuses __utimensat64_helper defined for __utimensat64.

The test procedure for __futimens64 is the same as for __utimensat64 conversion
patch.