RISC-V: Use 64-bit-time syscall numbers with the 32-bit port

sysdep.h redefines only the syscall where the generic implementation
still does not have actual 64-bit time_t support:

   /* Workarounds for generic code needing to handle 64-bit time_t.  */

   /* Fix sysdeps/unix/sysv/linux/clock_getcpuclockid.c.  */
   #define __NR_clock_getres    __NR_clock_getres_time64
   /* Fix sysdeps/nptl/lowlevellock-futex.h.  */
   #define __NR_futex           __NR_futex_time64
   [...]

This patch also adds a comment that it is a workaround to handle 64-bit
time_t and on each #define comment for which implementation it intends
to.

Reviewed-by: Maciej W. Rozycki <macro@wdc.com>