Set OpenBSD/powerpc cache sizes.

Index: util/cacheflush.c
--- util/cacheflush.c.orig
+++ util/cacheflush.c
@@ -167,6 +167,13 @@ static void arch_cache_info(int *isize, int *dsize)
     have_coherent_icache = qemu_getauxval(AT_HWCAP) & PPC_FEATURE_ICACHE_SNOOP;
 }
 
+#elif defined(_ARCH_PPC) && defined (__OpenBSD__)
+static void arch_cache_info(int *isize, int *dsize)
+{
+    *isize = 32;
+    *dsize = 32;
+}
+
 #else
 static void arch_cache_info(int *isize, int *dsize) { }
 #endif /* arch_cache_info */