Bug#758227: mpg123: Add support for arm64

[peter green]

>
> mpg123 currently does not understand that arm64 is a 64bit
> architecture causing FTBFS.
>
> The attached patch fixes the failure.
>   
I've just uploaded this patch to debian-ports arm64 unreleased.

Since we are currently in the process of trying to push arm64 into 
debian proper we would appreciate an upload to sid soonish.