riscv, kexec: fix the ifdeffery for AFLAGS_kexec_relocate.o

This was introduced in commit fba8a8674f68 ("RISC-V: Add kexec
support").

It should work on CONFIG_KEXEC_CORE, but not CONFIG_KEXEC only, since
we could set CONFIG_KEXEC_FILE=y and CONFIG_KEXEC=N, or only set
CONFIG_CRASH_DUMP=y and disable both CONFIG_KEXEC and CONFIG_KEXEC_FILE.
In these cases, the AFLAGS won't take effect with the current ifdeffery
for AFLAGS_kexec_relocate.o.

So fix it now.

Link: https://lkml.kernel.org/r/20231201062538.27240-1-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Nick Kossifidis <mick@ics.forth.gr>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>