linux: Add support for clock_gettime64 vDSO

No architecture currently defines the vDSO symbol.  On architectures
with 64-bit time_t the HAVE_CLOCK_GETTIME_VSYSCALL is renamed to
HAVE_CLOCK_GETTIME64_VSYSCALL, it simplifies clock_gettime code.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>