Adjust invocations of llvm-objdump

With llvm-13.0.0~rc1, we get the following error:

Command `'llvm-objdump --arch-name x86_64 --section __TEXT,__text --macho --demangle --no-leading-addr --no-show-raw-insn {}'` failed with exit code 1. Standard output:
│┄     llvm-objdump: error: unknown argument '--arch-name'

--arch-name=… works with both the new llvm and older ones.
I thought this would fix #275, but it turns out it's not enough.

diffoscope/comparators/macho.py

@@ -294,10 +294,8 @@ class LlvmObjdump(Command):
 
     def objdump_options(self):
         return [
-            "--arch-name",
-            self._arch,
-            "--section",
-            self._section,
+            f"--arch-name={self._arch}",
+            f"--section={self._section}",
             "--macho",
             "--demangle",
             "--no-leading-addr",