MIPS: O32: Provide definition of registers ta0 .. ta3.

Later IRIX versions provide them in <sys/regdef.h> and gas also accepts
$ta0 .. $ta3 since binutils 2.18 so Linux should do the same for source
compatibility.

arch/mips/include/asm/regdef.h

@@ -6,6 +6,8 @@
  * Copyright (C) 1985 MIPS Computer Systems, Inc.
  * Copyright (C) 1994, 95, 99, 2003 by Ralf Baechle
  * Copyright (C) 1990 - 1992, 1999 Silicon Graphics, Inc.
+ * Copyright (C) 2011 Wind River Systems,
+ *   written by Ralf Baechle <ralf@linux-mips.org>
  */
 #ifndef _ASM_REGDEF_H
 #define _ASM_REGDEF_H
@@ -30,9 +32,13 @@
 #define t2      $10
 #define t3      $11
 #define t4      $12
+#define ta0	$12
 #define t5      $13
+#define ta1	$13
 #define t6      $14
+#define ta2	$14
 #define t7      $15
+#define ta3	$15
 #define s0      $16     /* callee saved */
 #define s1      $17
 #define s2      $18