Add RISC-V support

Looks like that riscv support has been in upstream mailing list for a while, Ubuntu has those patches merged too, so I try to add these patches to Debian too.

build on amd64 and riscv64 with gcc-10 succeed, test on Unmatched and Allwinner D1, works fine.

However latest toolchain will make it unusable on riscv, when running grub-mkimage, it report "relocation 0x13 is not implemented yet." This problem only affect riscv, change back to old toolchain should make it work, and fix for this problem can be a new merge request.