build: enable support for aix/ppc64

* 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.

bin/cross-compile.go

@@ -74,6 +74,7 @@ var osarches = []string{
 	"plan9/amd64",
 	"solaris/amd64",
 	// "js/wasm", // Rclone is too big for js/wasm until https://github.com/golang/go/issues/64856 is fixed
+    "aix/ppc64",
 }
 
 // Special environment flags for a given arch