* Adds "aix/ppc64" to the cross-compile target list.
* Including AIX in the build tag of "metadata_other.go".
* Excluding AIX from the main ncdu build tags.
* Marking AIX as an unsupported platform for ncdu.
* Excluding AIX from the fallback redirect implementation.
* Excluding AIX from unix build tags to avoid undefined unix.WNOHANG.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.