kernel/arch/riscv64: fix crash on userland thread exit

Set first stack frame return address to
<commpage>commpage_thread_exit, so it will be called
when thread entry point returns.

Change-Id: Ide5cde8d4501eb7241e03ff4052174e984e78870
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4493
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

kernel/arch/commpage: implement for riscv64

Change-Id: Id7f495896a08a237db85dcedd1174e4c181e373d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4056
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

kernel: Fill remaining riscv64 architecture headers