Make THREAD_SIZE available to vmlinux.lds on cris by moving it to
asm/thread_info.h and including that from 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>
cc: Mikael Starvik <starvik@axis.com>
cc: Jesper Nilsson <jesper.nilsson@axis.com>
cc: linux-cris-kernel@axis.com