Add support for `atomic` construct

[scastd]

Describe the feature

The atomic construct has the following properties:

• Provides a special mechanism of mutual exclusion to do read and update operations to a specific storage location.
• Only supports simple read and update expressions (x += 1, x = x - foo();).
• Only supports the read and update part (foo() is not protected).

• NOT COMPATIBLE with critical:

  int x=1;
  
  #pragma omp parallel num_threads(2)
  {
  #pragma omp critical
  x++; // different threads can update x at the
  #pragma omp atomic
  x++; // same time!!!
  }
  
  printf("%d", x); // prints 2 or 3

Full OpenMP specification (link).

C code example:

int x=1;

#pragma omp parallel num_threads(2)
{
  #pragma omp atomic
  x++; // only 1 thread at a time updates x here
}

printf("%d", x); // prints 3

Additional information

• [x] Would you be willing to help implement this feature?

Final checks

• [X] Read the contribution guide.
• [X] Check existing discussions and issues.