pystone_lowmem: Lower default loop numbers to 500 (100 times less than was).

On 80MHz RISC system, that runs in ~2s. So, even on a 10MHz system, random
run should not make user wonder if a system hanged. Of course, for more
precise measurement, more loops should be used, e.g.:

pystone_lowmem.main(1000)

pystone_lowmem/pystone_lowmem.py

@@ -39,7 +39,7 @@ Version History:
 
 """
 
-LOOPS = 50000
+LOOPS = 500
 
 from utime import ticks_ms, ticks_diff