elf.h: Add RISC-V definitions

elf.h: Add RISC-V definitions

The RISC-V ABI document outlines ELF header structure and relocation
information. Pull the respective magic numbers into our elf header
so we can make use of them.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>