nios2: fix r15 issue for gcc4

The "-ffixed-r15" option doesn't work well for gcc4. Since we
don't use gp for small data with option "-G0", we can use gp
as global data pointer. This allows compiler to use r15. It
is necessary for gcc4 to work properly.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>
diff --git a/arch/nios2/include/asm/global_data.h b/arch/nios2/include/asm/global_data.h
index 34aa962..f1b3482 100644
--- a/arch/nios2/include/asm/global_data.h
+++ b/arch/nios2/include/asm/global_data.h
@@ -48,6 +48,6 @@
 #define	GD_FLG_LOGINIT	0x00020		/* Log Buffer has been initialized	*/
 #define GD_FLG_DISABLE_CONSOLE	0x00040		/* Disable console (in & out)	 */
 
-#define DECLARE_GLOBAL_DATA_PTR     register gd_t *gd asm ("r15")
+#define DECLARE_GLOBAL_DATA_PTR     register gd_t *gd asm ("gp")
 
 #endif /* __ASM_NIOS2_GLOBALDATA_H_ */