mips: enable decoding of set_thread_area

* linux/dummy.h [MIPS]: Do not redirect sys_set_thread_area to printargs.
* mem.c [MIPS] (sys_set_thread_area): Define.
diff --git a/linux/dummy.h b/linux/dummy.h
index 95efa8a..2fc3498 100644
--- a/linux/dummy.h
+++ b/linux/dummy.h
@@ -47,7 +47,9 @@
 # define	sys_modify_ldt		printargs
 # ifndef M68K
 #  define	sys_get_thread_area	printargs
-#  define	sys_set_thread_area	printargs
+#  ifndef MIPS
+#   define	sys_set_thread_area	printargs
+#  endif
 # endif
 #endif