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.