[parity-util-mem] remove cfg(target_arch) statements

[ordian]

As mentioned in paritytech/substrate#6745, this is not future-proof and leads to runtime panics.

Instead, if there is no way to compute malloc_size_of, we should either make it a compile-time/link-time error, or change the API to return an Option<usize> instead of usize (although, this may be a substantial rewrite).

I don't have time to work on this today, but feel free to ask me any questions.

cc @expenses , @cheme