openrisc: Add _text symbol to fix ksym build error

The build robot reports:

   .tmp_kallsyms1.o: In function `kallsyms_relative_base':
>> (.rodata+0x8a18): undefined reference to `_text'

This is when using 'make alldefconfig'. Adding this _text symbol to mark
the start of the kernel as in other architecture fixes this.
Signed-off-by: Stafford Horne <shorne@gmail.com>
Acked-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/kernel/vmlinux.lds.S

@@ -44,6 +44,8 @@ SECTIONS
         /* Read-only sections, merged into text segment: */
         . = LOAD_BASE ;
 
+	_text = .;
+
 	/* _s_kernel_ro must be page aligned */
 	. = ALIGN(PAGE_SIZE);
 	_s_kernel_ro = .;