Add environment variables to control loop unroll

[fmayer]

e.g.

export ALIVECC_SRC_UNROLL=10
export ALIVECC_TGT_UNROLL=10

That allows alivecc to verify -O2 for

int main() {
  for (int i = 0; i < 2; ++i) {
    if (i == 3) return 1;
  }
  return 2;
}