RISC-V: Prepare dropping week attribute from arch_kexec_apply_relocations[_add]

Without this change arch/riscv/kernel/elf_kexec.c fails to compile once
commit 233c1e6c319c ("kexec_file: drop weak attribute from
arch_kexec_apply_relocations[_add]") is also contained in the tree.
This currently happens in next-20220527.

Prepare the RISC-V similar to the s390 adaption done in 233c1e6c319c.
This is safe to do on top of the riscv change even without the change to
arch_kexec_apply_relocations.

Fixes: 838b3e28 ("RISC-V: Load purgatory in kexec_file")
Looks-good-to: liaochang (A) <liaochang1@huawei.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>