Problem with passing variable/register arguments to function

### Description

Whenever I try to call a function with a register argument, I get the NotImplementedError exception. Based on the error code, I'm assuming that this is because gadgets like `jmp rax` for function calls are not supported.

This code
```
rop.func_call('mprotect', ('rax', 0x1000, 0x7))
```

causes this exception

```
NotImplementedError: currently only support jmp_mem type func_jmp_gadgets!
```