microblaze: include EXIT_TEXT to _stext

Microblaze wants to throw out EXIT_TEXT during runtime too.  This
hasn't caused trouble till now because the linker script didn't
discard EXIT_TEXT and it ended up in its default output section.  As
discard definition is about to be unified, include EXIT_TEXT into
_stext explicitly and while at it replace explicit exitcall definition
to EXIT_CALL.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Tejun Heo <tj@kernel.org>