riscv64: Fill in some missing CPU defines, advance build further

Change-Id: Id050fad59ede444f2eab7eca681c6ec44612aaf9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3160
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: François Revol <revol@free.fr>