mention mips64 n32 ABI support in INSTALL doc

INSTALL

@@ -63,7 +63,7 @@ and ABI combinations:
       is required
 
 * MIPS64
-    * ABI is n64 (LP64)
+    * ABI is n64 (LP64) or n32 (ILP32)
     * Big-endian default; little-endian variants also supported
     * Default ABI variant uses FPU registers; alternate soft-float ABI
       that does not use FPU registers or instructions is available