OvmfPkg/RiscVVirt: Add build files for Qemu Virt platform

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4076



Add infrastructure files to build edk2 for RISC-V qemu virt machine.

- It follows PEI less design.
- EDK2 for qemu virt is booted in S-mode as a payload for M-mode FW
- Leveraged from ArmVirtQemu

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Abner Chang <abner.chang@amd.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>