riscv: compat: Add elf.h implementation

Implement necessary type and macro for compat elf. See the code
comment for detail.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-15-guoren@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>