ARM64 backend, part 2 / 11: remove old ARM64 backend.

This removes the old ARM64 backend completely, leaving only an empty
`arm64` module. The tree at this state will not build with the `arm64`
feature enabled, but that feature has to be enabled explicitly (it is
not default). Subsequent patches will fill in the new backend.

cranelift/codegen/src/isa/mod.rs

@@ -116,7 +116,6 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
             isa_builder!(x86, "x86", triple)
         }
         Architecture::Arm { .. } => isa_builder!(arm32, "arm32", triple),
-        Architecture::Aarch64 { .. } => isa_builder!(arm64, "arm64", triple),
         _ => Err(LookupError::Unsupported),
     }
 }