conform to 6.4.4.4 for hex and octal escapes

fixes problem noted in #2610

test/val/bug2610.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+#if '\x0A' != 0x0A
+#error "Suspicious character set translation"
+#endif
+int main()
+{
+    char c = '\x0A';
+    if (c == 0x0A) {
+        printf("Ok\n");
+        return 0;
+    } else {
+        printf("Failed\n");
+        return 1;
+    }
+}