linux-user: switch default ppc64 CPU to 970fx from 970

Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit f7177937)

linux-user/main.c

@@ -2684,7 +2684,7 @@ int main(int argc, char **argv, char **envp)
 #endif
 #elif defined(TARGET_PPC)
 #ifdef TARGET_PPC64
-        cpu_model = "970";
+        cpu_model = "970fx";
 #else
         cpu_model = "750";
 #endif