kernel/arch/thread: implement for riscv64

Change-Id: I3effa598626b32c29606299cd0edee390d430baf
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4066
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

headers/riscv64: Implement basic arch headers

Change-Id: I6bfbacb61eae84ffebc30c2565683348d684d88f
Reviewed-on: https://review.haiku-os.org/c/1063
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>