y2038: linux: Provide __ntp_gettime64 implementation

This patch provides new __ntp_gettime64 explicit 64 bit function for getting
time parameters via NTP interface.

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 - __ntp_gettime has been refactored to internally
use __ntp_gettime64.

The __ntp_gettime is now supposed to be used on systems still supporting 32
bit time (__TIMESIZE != 64) - hence the necessary conversions between struct
ntptimeval and 64 bit struct __ntptimeval64.

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 __ntp_gettime64 and __ntp_gettime.

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

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

@@ -33,6 +33,7 @@ libc_hidden_proto (__adjtimex)
 #   define __clock_adjtime64 __clock_adjtime
 #   define ___adjtimex64 ___adjtimex
 #   define __ntptimeval64 ntptimeval
+#   define __ntp_gettime64 __ntp_gettime
 #  else
 
 struct __timex64
@@ -91,6 +92,9 @@ struct __ntptimeval64
   long int __glibc_reserved3;
   long int __glibc_reserved4;
 };
+extern int __ntp_gettime64 (struct __ntptimeval64 *ntv);
+libc_hidden_proto (__ntp_gettime64)
+
 #  endif
 
 /* Convert a known valid struct timex into a struct __timex64.  */