CRIS: Add _sdata to vmlinux.lds.S

Fixes link error:
  LD      vmlinux
kernel/built-in.o: In function `core_kernel_data':
(.text+0x13e44): undefined reference to `_sdata'
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>

arch/cris/kernel/vmlinux.lds.S

@@ -52,6 +52,7 @@ SECTIONS
 
 	EXCEPTION_TABLE(4)
 
+	_sdata = .;
 	RODATA
 
 	. = ALIGN (4);