Fix traceback on AIX in shlib archiving code

A compiler may not have a linker:
```
'NoneType' object has no attribute 'get_command_to_archive_shlib'
```

mesonbuild/compilers/compilers.py

@@ -962,6 +962,8 @@ class Compiler(HoldableObject, metaclass=abc.ABCMeta):
         return self.linker.get_archive_name(filename)
 
     def get_command_to_archive_shlib(self) -> T.List[str]:
+        if not self.linker:
+            return []
         return self.linker.get_command_to_archive_shlib()
 
     def thread_flags(self, env: 'Environment') -> T.List[str]: