doc/1.12: add notes about aix/ppc64 port

Fixes #29315

Change-Id: I6ecc5109c23e7a7d9db54250bf041acc841701e3
Reviewed-on: https://go-review.googlesource.com/c/154697Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -68,7 +68,7 @@ Go 1.13 will require macOS 10.11 El Capitan or later.
 <h3 id="aix">AIX</h3>
 
 <p>
-  TODO: status of AIX port?
+  Go now supports AIX 7.2 and later on POWER8 architectures (<code>aix/ppc64</code>). External linking, cgo, pprof and the race detector aren't yet supported.
 </p>
 
 <h3 id="hurd">Hurd</h3>