java/interp: Use std::vector for the operand stack

Test case:

  [penberg@nero hornet]$ cat Test.java
  public class Test {
   public static int sum(int x, int y) {
      return x+y;
    }

    public static void main(String[] args) {
      for (int i = 0; i < 10000000; i++) {
        sum(1, 2);
      }
    }
  }

[penberg@nero hornet]$ make && perf stat ./hornet Test
make: Nothing to be done for `all'.
warning: java/lang/Object::registerNatives()V: stubbed
warning: java/lang/Object::registerNatives()V: stubbed

 Performance counter stats for './hornet Test':

       1613.522294      task-clock (msec)         #    0.998 CPUs utilized
                 5      context-switches          #    0.003 K/sec
                 2      cpu-migrations            #    0.001 K/sec
             2,051      page-faults               #    0.001 M/sec
     6,411,134,532      cycles                    #    3.973 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    19,064,039,101      instructions              #    2.97  insns per cycle
     3,932,237,183      branches                  # 2437.052 M/sec
           195,646      branch-misses             #    0.00% of all branches

       1.616258380 seconds time elapsed

[penberg@nero hornet]$ make && perf stat ./hornet Test
make: Nothing to be done for `all'.
warning: java/lang/Object::registerNatives()V: stubbed
warning: java/lang/Object::registerNatives()V: stubbed

 Performance counter stats for './hornet Test':

       1060.019187      task-clock (msec)         #    0.998 CPUs utilized
                 6      context-switches          #    0.006 K/sec
                 0      cpu-migrations            #    0.000 K/sec
             2,054      page-faults               #    0.002 M/sec
     4,171,724,095      cycles                    #    3.936 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    10,967,041,439      instructions              #    2.63  insns per cycle
     2,356,901,456      branches                  # 2223.452 M/sec
         5,594,541      branch-misses             #    0.24% of all branches

       1.061896833 seconds time elapsed

Signed-off-by: Pekka Enberg <penberg@iki.fi>