sysdeps: Add clock_gettime64 vDSO

Add support for the clock_gettim64 vDSO calls. These are protected by
the HAVE_CLOCK_GETTIME64_VSYSCALL define.

HAVE_CLOCK_GETTIME64_VSYSCALL should be defined for 32-bit platforms
(WORDSIZE == 32) that only run on the 5.1 kernel or later. WORDSIZE ==
64 platforms can use #define __vdso_clock_gettime64 __vdso_clock_gettime
and use the __vdso_clock_gettime syscall as they don't have a
__vdso_clock_gettime64 call.

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