s390/kernel: move EX_TABLE macros to linkage.h header file

Move the EX_TABLE macro definitions from the processor.h to the linkage.h
header file.  It helps to reduce circular header file dependencies.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>