sh: Add KEEP order to start.o section

The start.o section is changed by --gc-section option of ld.
Of this using KEEP order, therefore, evade this problem.
Signed-off-by: NNobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: NNobuhiro Iwamatsu <iwamatsu@nigauri.org>

arch/sh/cpu/sh2/u-boot.lds

@@ -38,7 +38,7 @@ SECTIONS
 
 	.text :
 	{
-		arch/sh/cpu/sh2/start.o		(.text)
+		KEEP(arch/sh/cpu/sh2/start.o	(.text))
 		. = ALIGN(8192);
 		common/env_embedded.o	(.ppcenv)
 		. = ALIGN(8192);

arch/sh/cpu/sh3/u-boot.lds

@@ -44,7 +44,7 @@ SECTIONS
 
 	.text :
 	{
-		arch/sh/cpu/sh3/start.o		(.text)
+		KEEP(arch/sh/cpu/sh3/start.o	(.text))
 		. = ALIGN(8192);
 		common/env_embedded.o	(.ppcenv)
 		. = ALIGN(8192);

arch/sh/cpu/sh4/u-boot.lds

@@ -41,7 +41,7 @@ SECTIONS
 
 	.text :
 	{
-		arch/sh/cpu/sh4/start.o		(.text)
+		KEEP(arch/sh/cpu/sh4/start.o		(.text))
 		. = ALIGN(8192);
 		common/env_embedded.o	(.ppcenv)
 		. = ALIGN(8192);