hexagon: Make THREAD_SIZE available to vmlinux.lds

Make THREAD_SIZE available to vmlinux.lds on hexagon 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>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Richard Kuo <rkuo@codeaurora.org>
cc: linux-hexagon@vger.kernel.org