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.

cmd/ncdu/ncdu.go

@@ -1,4 +1,4 @@
-//go:build !plan9 && !js
+//go:build !plan9 && !js && !aix
 
 // Package ncdu implements a text based user interface for exploring a remote
 package ncdu

cmd/ncdu/ncdu_unsupported.go

@@ -1,7 +1,7 @@
 // Build for ncdu for unsupported platforms to stop go complaining
 // about "no buildable Go source files "
 
-//go:build plan9 || js
+//go:build plan9 || js || aix
 
 // Package ncdu implements a text based user interface for exploring a remote
 package ncdu