time-util: fall back to CLOCK_MONOTONIC if CLOCK_BOOTTIME unsupported

It was added in 2.6.39, and causes an assertion to fail when running in mock
hosted on 2.6.23-based RHEL-6:

Assertion 'clock_gettime(map_clock_id(clock_id), &ts) == 0' failed at systemd/src/basic/time-util.c:70, function now(). Aborting.

src/basic/time-util.c

@@ -47,12 +47,15 @@ static clockid_t map_clock_id(clockid_t c) {
         /* Some more exotic archs (s390, ppc, …) lack the "ALARM" flavour of the clocks. Thus, clock_gettime() will
          * fail for them. Since they are essentially the same as their non-ALARM pendants (their only difference is
          * when timers are set on them), let's just map them accordingly. This way, we can get the correct time even on
-         * those archs. */
+         * those archs.
+         *
+         * Also, older kernels don't support CLOCK_BOOTTIME: fall back to CLOCK_MONOTONIC. */
 
         switch (c) {
 
+        case CLOCK_BOOTTIME:
         case CLOCK_BOOTTIME_ALARM:
-                return CLOCK_BOOTTIME;
+                return clock_boottime_or_monotonic ();
 
         case CLOCK_REALTIME_ALARM:
                 return CLOCK_REALTIME;