linkers: fix regression when using lld after iOS changes

```
$ LDFLAGS="-fuse-ld=lld" meson setup builddir/
[...]
    linker = lld_cls(
        compiler, for_machine, comp_class.LINKER_PREFIX, override, system=system, version=v)
TypeError: LLVMDynamicLinker.__init__() got an unexpected keyword argument 'system'
```

Fixes regression in commit cece1a7. We
pass system=system to the linker construction, which worked fine for
Apple LLVM LD64 as that inherited `__init__` from DynamicLinker. But
non-Apple LLD had an overridden `__init__` which wasn't adapted.

Even if it is thrown away and never used, since LLVM isn't an Apple
linker, we still need to obey the argument contract.

Author: eli-schwartz

mesonbuild/linkers/linkers.py

@@ -897,8 +897,9 @@ class LLVMDynamicLinker(GnuLikeDynamicLinkerMixin, PosixDynamicLinkerMixin, Dyna
 
     def __init__(self, exelist: T.List[str],
                  for_machine: mesonlib.MachineChoice, prefix_arg: T.Union[str, T.List[str]],
-                 always_args: T.List[str], *, version: str = 'unknown version'):
-        super().__init__(exelist, for_machine, prefix_arg, always_args, version=version)
+                 always_args: T.List[str], *, system: str = 'unknown system',
+                 version: str = 'unknown version'):
+        super().__init__(exelist, for_machine, prefix_arg, always_args, system=system, version=version)
 
         # Some targets don't seem to support this argument (windows, wasm, ...)
         self.has_allow_shlib_undefined = self._supports_flag('--allow-shlib-undefined', always_args)