riscv/kexec_file: Fix relocation type R_RISCV_ADD16 and R_RISCV_SUB16 unknown

Runs on the kernel with CONFIG_RISCV_ALTERNATIVE enabled:
  kexec -sl vmlinux

Error:
  kexec_image: Unknown rela relocation: 34
  kexec_image: Error loading purgatory ret=-8
and
  kexec_image: Unknown rela relocation: 38
  kexec_image: Error loading purgatory ret=-8

The purgatory code uses the 16-bit addition and subtraction relocation
type, but not handled, resulting in kexec_file_load failure.
So add handle to arch_kexec_apply_relocations_add().

Tested on RISC-V64 Qemu-virt, issue fixed.
Co-developed-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Ying Sun <sunying@isrc.iscas.ac.cn>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240711083236.2859632-1-sunying@isrc.iscas.ac.cnSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/elf_kexec.c

@@ -451,6 +451,12 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 			*(u32 *)loc = CLEAN_IMM(CJTYPE, *(u32 *)loc) |
 				 ENCODE_CJTYPE_IMM(val - addr);
 			break;
+		case R_RISCV_ADD16:
+			*(u16 *)loc += val;
+			break;
+		case R_RISCV_SUB16:
+			*(u16 *)loc -= val;
+			break;
 		case R_RISCV_ADD32:
 			*(u32 *)loc += val;
 			break;