y2038: Introduce the __ASSUME_TIME64_SYSCALLS define

Add a macro to linux/kernel-features.h, __ASSUME_TIME64_SYSCALLS, to
indicate whether the kernel can be assumed to provide a set of system
calls that process 64-bit time_t.

__ASSUME_TIME64_SYSCALLS does not indicate whether time_t is actually
64 bits (that's __TIMEBITS) and also does not indicate whether the
64-bit time_t system calls have "time64" suffixes on their names.

Code that uses __ASSUME_TIME64_SYSCALLS will be added in subsequent
patches.

	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_TIME64_SYSCALLS): New macro.

Reviewed-by: Alistair Francis <alistair23@gmail.com>
Reviewed-by: Joseph Myers <joseph@codesourcery.com>
Reviewed-by: Zack Weinberg <zackw@panix.com>