samples: llext shell_loader reduced size

Reduce the heap and stack sizes needed for the shell loader sample app
so more devices may try it out.

Tried on a mimxrt1010_evk and noted the total ram usage was
approximately 44000 bytes. Prior to this change the mimxrt1010_evk was
unable to run the sample as the bss section overflowed the ram size.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>