grub: Add support for RISC-V

grub: Add support for RISC-V

This patch adds RISC-V to the COMPATIBLE_HOST. Since GRUB 2.04,
the source code supports the RISC-V, thanks to Alexander Graf.

Adding the GRUBPLATFORM for RISC-V prevents autoconf problems.
Also, the patch appends the __anonymous method with RISC-V architecture.

Signed-off-by: Norbert Kaminski <norbert.kaminski@3mdeb.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>