[PowerPC] fix default cpu setting

Summary:
for platform that returns nothing for getHostCPUName()
For example for target ARM on windows. For this case, -mcpu=native
should set CPU to the default according to triple instead of setting
CPU to "native"

Fixes https://lab.llvm.org/buildbot/#/builders/161/builds/873 caused by
#97541

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60250515

Author: Chen Zheng

llvm/lib/TargetParser/PPCTargetParser.cpp

@@ -97,7 +97,7 @@ StringRef getNormalizedPPCTargetCPU(const Triple &T, StringRef CPUName) {
     }
 
     StringRef CPU = normalizeCPUName(CPUName);
-    if (CPU != "generic")
+    if (CPU != "generic" && CPU != "native")
       return CPU;
   }