fix wchar limits mistakenly copied from arm

arch/powerpc/bits/wchar.h

@@ -1,4 +1,4 @@
 #ifndef WCHAR_MIN
-#define WCHAR_MIN 0U
-#define WCHAR_MAX 0xffffffffU
+#define WCHAR_MIN (-1-0x7fffffff)
+#define WCHAR_MAX (0x7fffffff)
 #endif