MN10300: Add missing _sdata declaration

_sdata needs to be declared in the linker script now as of commit
a2d063ac ("extable, core_kernel_data(): Make sure all archs define
_sdata")

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>