add missing errno's for the platform abstraction layer

Index: external/corefx/src/Native/Unix/System.Native/pal_errno.c
--- external/corefx/src/Native/Unix/System.Native/pal_errno.c.orig
+++ external/corefx/src/Native/Unix/System.Native/pal_errno.c
@@ -13,6 +13,19 @@
 #define ENODATA ENOATTR
 #endif
 
+#if defined(__OpenBSD__)
+#if !defined(EMULTIHOP)
+#define EMULTIHOP 96 // outside of sys/errno.h's range to avoid conflict
+#endif
+#if !defined(ENODATA)
+#define ENODATA 97 // outside of sys/errno.h's range to avoid conflict
+#endif
+#if !defined(ENOLINK)
+#define ENOLINK 98 // outside of sys/errno.h's range to avoid conflict
+#endif
+#endif /*__OpenBSD__*/
+
+
 #include <string.h>
 #include <assert.h>
 
