Using +tcharm_stacksize option to set larger/smaller stack size for your AMPI VPs.

Experiment 1: In hello.c, specify size of bigArray to be 10000, so that the program
takes about 10KB stack space. Build and run it with +vp2000. With 2000 VPs with 
default stack size of 1MB, the virtual memory address available to user (2GB) will run 
up and the program crashes. Now run with +tcharm_stacksize=100000 to give only 100KB
to each VP, and the +vp2000 run can return with success.

Experiment 2: This time set the size of bigArray to 2000000, and the memory footprint
of this program should be over 2MB, which is larger than the default size. Do a run 
with +vp8 and you'll find charmrun cannot handle this. We have to give VPs larger 
stack size. If we try with +tcharm_stacksize=10000000, it should return with success. 
