Check for the only 32-bit MIPS ABIs we support, rather than !n64

There may be additional 64-bit ABIs supported, so use a positive check rather
than a negative check.
Suggested by:	imp
MFC after:	1 week
Sponsored by:	Juniper Networks, Inc

sys/mips/include/elf.h

@@ -105,7 +105,7 @@ typedef struct {	/* Auxiliary vector entry on initial stack */
 	int	a_type;			/* Entry type. */
 	union {
 		int	a_val;		/* Integer value. */
-#ifndef __mips_n64
+#if defined(__mips_o32) || defined(__mips_n32)
 		void	*a_ptr;		/* Address. */
 		void	(*a_fcn)(void); /* Function pointer (not used). */
 #endif