Make THREAD_SIZE available to vmlinux.lds on openrisc by including
asm/thread_info.h the linker script.

This allows init_stack to be allocated in the linker script in a subsequent
patch.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Stafford Horne <shorne@gmail.com>
cc: Jonas Bonn <jonas@southpole.se>
cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
cc: openrisc@lists.librecores.org