s390/kexec: make machine_kexec() depend on CONFIG_KEXEC_CORE

Make machine_kexec.o and relocate_kernel.o depend on
CONFIG_KEXEC_CORE option as other architectures do.

Still generate machine_kexec_reloc.o unconditionally,
since arch_kexec_do_relocs() function is neded by the
decompressor.

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>