s390: Fix build when using EXEEXT_FOR_BUILD

Suffix the s390-mkopc build utility executable file name with
EXEEXT_FOR_BUILD. Otherwise it cannot be located when building with
EXEEXT_FOR_BUILD. Use pattern used for other architecture build
utilities and compile and link s390-mkopc in two steps.

While at it also specify the dependencies of s390-mkopc.c.

opcodes/
	* Makefile.am: Add target to build s390-mkopc.o. Correct
	  target to build s390-mkopc$(EXEEXT_FOR_BUILD).
	* Makefile.in: Regenerate.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>