y2038: linux: Provide __clock_adjtime64 implementation

This patch replaces auto generated wrapper (as described in
sysdeps/unix/sysv/linux/syscalls.list) for clock_adjtime with one which adds
extra support for reading 64 bit time values on machines with __TIMESIZE != 64.

To achieve this goal new __clock_adjtime64 explicit 64 bit function for
adjusting Linux clock has been added.
Moreover, a 32 bit version - __clock_adjtime has been refactored to internally
use __clock_adjtime64.

The __clock_adjtime is now supposed to be used on systems still supporting 32
bit time (__TIMESIZE != 64) - hence the necessary conversions between 64 bit
struct __timespec64 and struct timespec.

The new __clock_adjtime64 syscall available from Linux 5.1+ has been used, when
applicable.
Up till v5.4 in the Linux kernel there was a bug preventing this call from
obtaining correct struct's timex time.tv_sec time after time_t overflow
(i.e. not being Y2038 safe).

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

Linux kernel, headers and minimal kernel version for glibc build test matrix:
- Linux v5.1 (with clock_adjtime64) and glibc build with v5.1 as
  minimal kernel version (--enable-kernel="5.1.0")
  The __ASSUME_TIME64_SYSCALLS flag defined.

- Linux v5.1 and default minimal kernel version
  The __ASSUME_TIME64_SYSCALLS not defined, but kernel supports clock_adjtime64
  syscall.

- Linux v4.19 (no clock_adjtime64 support) with default minimal kernel version
  for contemporary glibc (3.2.0)
  This kernel doesn't support clock_adjtime64 syscall, so the fallback to
  clock_adjtime is tested.

Above tests were performed with Y2038 redirection applied as well as without
(so the __TIMESIZE != 64 execution path is checked as well).

No regressions were observed.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/include/sys/timex.h

@@ -30,6 +30,7 @@ libc_hidden_proto (__adjtimex)
 /* Local definition of 64 bit time supporting timex struct */
 #  if __TIMESIZE == 64
 #   define __timex64 timex
+#   define __clock_adjtime64 __clock_adjtime
 #  else
 
 struct __timex64
@@ -71,6 +72,8 @@ struct __timex64
   int  :32;
   int  :32;
 };
+extern int __clock_adjtime64 (const clockid_t clock_id, struct __timex64 *tx64);
+libc_hidden_proto (__clock_adjtime64);
 #  endif
 
 /* Convert a known valid struct timex into a struct __timex64.  */