y2038: Make CONFIG_64BIT_TIME unconditional

As Stepan Golosunov points out, there is a small mistake in the
get_timespec64() function in the kernel. It was originally added under the
assumption that CONFIG_64BIT_TIME would get enabled on all 32-bit and
64-bit architectures, but when the conversion was done, it was only turned
on for 32-bit ones.

The effect is that the get_timespec64() function never clears the upper
half of the tv_nsec field for 32-bit tasks in compat mode. Clearing this is
required for POSIX compliant behavior of functions that pass a 'timespec'
structure with a 64-bit tv_sec and a 32-bit tv_nsec, plus uninitialized
padding.

The easiest fix for linux-5.1 is to just make the Kconfig symbol
unconditional, as it was originally intended. As a follow-up, the #ifdef
CONFIG_64BIT_TIME can be removed completely..

Note: for native 32-bit mode, no change is needed, this works as
designed and user space should never need to clear the upper 32
bits of the tv_nsec field, in or out of the kernel.

Fixes: 00bf25d6 ("y2038: use time32 syscall names on 32-bit")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Joseph Myers <joseph@codesourcery.com>
Cc: libc-alpha@sourceware.org
Cc: linux-api@vger.kernel.org
Cc: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Stepan Golosunov <stepan@golosunov.pp.ru>
Link: https://lore.kernel.org/lkml/20190422090710.bmxdhhankurhafxq@sghpc.golosunov.pp.ru/
Link: https://lkml.kernel.org/r/20190429131951.471701-1-arnd@arndb.de