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>