Make the hardclock test optional

Known to fail on VMs (such as the buildbots), see eg
http://blog.badtrace.com/post/rdtsc-x86-instruction-to-detect-vms/

include/mbedtls/timing.h

@@ -62,6 +62,10 @@ extern volatile int mbedtls_timing_alarmed;
 
 /**
  * \brief          Return the CPU cycle counter value
+ *
+ * \warning        This is only a best effort! Do not rely on this!
+ *                 In particular, it is known to be unreliable on virtual
+ *                 machines.
  */
 unsigned long mbedtls_timing_hardclock( void );