Add AIX triple to Cargo.toml for doc.

Author: Xing Xue

Cargo.toml

@@ -73,6 +73,7 @@ targets = [
     "powerpc-unknown-netbsd",
     "powerpc-wrs-vxworks",
     "powerpc-wrs-vxworks-spe",
+    "powerpc64-ibm-aix",
     "powerpc64-unknown-freebsd",
     "powerpc64-unknown-linux-gnu",
     "powerpc64-wrs-vxworks",