Change stub example to use asm-generic/sections.h

We can use the declarations of __bss_start and _end from this header
instead of declaring them locally.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/include/asm/u-boot-x86.h b/arch/x86/include/asm/u-boot-x86.h
index 948615d..3e380fe 100644
--- a/arch/x86/include/asm/u-boot-x86.h
+++ b/arch/x86/include/asm/u-boot-x86.h
@@ -25,13 +25,13 @@
 #define _U_BOOT_I386_H_	1
 
 /* Exports from the Linker Script */
-extern ulong __text_start;
+extern char __text_start[];
 extern ulong __data_end;
 extern ulong __rel_dyn_start;
 extern ulong __rel_dyn_end;
-extern ulong __bss_start;
+extern char __bss_start[];
 extern ulong __bss_end;
-extern ulong _end;
+extern char _end[];
 
 /* cpu/.../cpu.c */
 int x86_cpu_init_r(void);