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

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/linux-user/main.c b/linux-user/main.c
index ca49cc4..b394c00 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -2788,7 +2788,7 @@
 #endif
 #elif defined(TARGET_PPC)
 #ifdef TARGET_PPC64
-        cpu_model = "970";
+        cpu_model = "970fx";
 #else
         cpu_model = "750";
 #endif