`ArchPcode` doesn't return the right `ret_offset`

### Description

```py
from archinfo import ArchPcode
arch = ArchPcode("MIPS:LE:32:default")
print(arch.register_names[arch.ret_offset])
'f0'
```

While the result should be `v0`.

I think it because in the `.cspec` file, we can see that `f0_1` comes before `v0`
```xml

```

I think a quick fix will be to select the tag that doesn't have `metatype` value, but I am not sure if its the right solution

### Steps to reproduce the bug

_No response_

### Environment

_No response_

### Additional context

_No response_