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.

fs/log/redirect_stderr.go

@@ -1,6 +1,6 @@
 // Log the panic to the log file - for oses which can't do this
 
-//go:build !windows && !darwin && !dragonfly && !freebsd && !linux && !nacl && !netbsd && !openbsd
+//go:build !windows && !darwin && !dragonfly && !freebsd && !linux && !nacl && !netbsd && !openbsd && !aix
 
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