Changelog:
**Bugfix**:
* fix `find_function_addr` not implemented for `CleLoader`
* fix warning message of unsupported functions when using `CleLoader`
**Features**:
* add `ProcessState.is_syscall()` to check if the current instruction is a syscall
instruction (in the target architecture)
* add `open` function symbolic routine.
**Behavior changes**:
* a call to `ProcessState.fetch_instruction` will by default position the `current_instruction` to the one being fetched.
* a warning message now printed when executing a syscall