y2038: linux: Provide __adjtime64 implementation

This patch provides new __adjtime64 explicit 64 bit function for adjusting
Linux kernel clock.

Internally, the __clock_adjtime64 syscall is used instead of __adjtimex. This
patch is necessary for having architectures with __WORDSIZE == 32 Y2038 safe.

Moreover, a 32 bit version - __adjtime has been refactored to internally use
__adjtime64.

The __adjtime is now supposed to be used on systems still supporting 32
bit time (__TIMESIZE != 64) - hence the necessary conversions between struct
timeval and 64 bit struct __timeval64.

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Run-time tests:
- Run specific tests on ARM/x86 32bit systems (qemu):
  https://github.com/lmajewski/meta-y2038 and run tests:
  https://github.com/lmajewski/y2038-tests/commits/master

Above tests were performed with Y2038 redirection applied as well as without to
test the proper usage of both __adjtime64 and __adjtime.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

include/sys/time.h

@@ -26,6 +26,15 @@ extern int __settimezone (const struct timezone *__tz)
 	attribute_hidden;
 extern int __adjtime (const struct timeval *__delta,
 		      struct timeval *__olddelta);
+
+#  include <struct___timeval64.h>
+#  if __TIMESIZE == 64
+#   define __adjtime64 __adjtime
+#  else
+extern int __adjtime64 (const struct __timeval64 *itv,
+                        struct __timeval64 *otv);
+libc_hidden_proto (__adjtime64)
+#  endif
 extern int __getitimer (enum __itimer_which __which,
 			struct itimerval *__value);
 extern int __setitimer (enum __itimer_which __which,