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>

grub-core/kern/emu/lite.c

@@ -24,6 +24,8 @@
 #elif defined(__aarch64__)
 #include "../arm64/dl_helper.c"
 #include "../arm64/dl.c"
+#elif defined(__riscv)
+#include "../riscv/dl.c"
 #else
 #error "No target cpu type is defined"
 #endif