RISC-V: Use common linux loader

RISC-V: Use common linux loader

RISC-V doesn't have to do anything very different from other architectures
to loader EFI stub linux kernel. As a result, just use the common linux
loader instead of defining a RISC-V specific linux loader.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>