openrisc: Support both old (or32) and new (or1k) toolchain

The output file format for or1k has changed from "elf32-or32"
to "elf32-or1k". Select the correct output format automatically
to be able to compile the kernel with both toolchain variants.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Stafford Horne <shorne@gmail.com>