Dynamically allocate scudo allocation buffer.

This is so we can increase the buffer size for finding elusive bugs.

Tested by hand with this program

```

int main(int argc, char** argv) {
  if (argc < 2)
    return 1;
  int n = atoi(argv[1]);
  char* x = reinterpret_cast<char*>(malloc(1));
  *((volatile char*)x) = 1;
  free(x);
  for (; n > 0; --n) {
    char* y = reinterpret_cast<char*>(malloc(1024));
    *((volatile char*)y) = 1;
    free(y);
  }
  *x = 2;
  return 0;
}
```

SCUDO_OPTIONS=allocation_ring_buffer_size=30000 ./uaf 1000000
-> no allocation trace
SCUDO_OPTIONS=allocation_ring_buffer_size=30000000 ./uaf 1000000
-> allocation trace

Reviewed By: hctim, eugenis

Differential Revision: https://reviews.llvm.org/D140932