RISC-V: Add EM_RISCV to kexec UAPI header

Add RISC-V to the list of supported kexec architectures, we need to
add the definition early-on so that later patches can use it.

EM_RISCV is 243 as per ELF psABI specification here:
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md



Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>