[Power9] Add support for -mcpu=pwr9 in the back end

This patch corresponds to review:
http://reviews.llvm.org/D19683

Simply adds the bits for being able to specify -mcpu=pwr9 to the back end.

llvm-svn: 268950
diff --git a/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp b/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
index ac9d19c..ecf184e 100644
--- a/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
+++ b/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp
@@ -1306,8 +1306,10 @@
     "power6",
     "power6x",
     "power7",
+    // FIXME: why is power8 missing here?
     "ppc64",
-    "ppc64le"
+    "ppc64le",
+    "power9"
   };
 
   // Get the numerically largest directive.