RISC-V: Add to build system

RISC-V: Add to build system

This patch adds support for RISC-V to the grub build system. With this
patch, I can successfully build grub on RISC-V as a UEFI application.

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>