m68k, kexec: fix the incorrect ifdeffery and build dependency of CONFIG_KEXEC

The select of KEXEC for CRASH_DUMP in kernel/Kconfig.kexec will be
dropped, then compiling errors will be triggered if below config items are
set:

===
CONFIG_CRASH_CORE=y
CONFIG_KEXEC_CORE=y
CONFIG_CRASH_DUMP=y
===

Here, change the dependency of buinding machine_kexec.o relocate_kernel.o
and the ifdeffery in asm/kexe.h to CONFIG_KEXEC_CORE.

Link: https://lkml.kernel.org/r/20231208073036.7884-3-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Eric DeVolder <eric_devolder@yahoo.com>
Cc: Ignat Korchagin <ignat@cloudflare.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>