riscv: module: Add ADD16 and SUB16 rela types

To prepare for 16-bit relocation types to be emitted in alternatives
add support for ADD16 and SUB16.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230128172856.3814-9-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>