kernel: add missing support of simple signal handler for riscv64

Change-Id: Iab9d09344a99013b381b64d5438ded0a083b3a52
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5202
Reviewed-by: Rene Gollent <rene@gollent.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

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>

riscv64: Fill in more bulk architecture items around libroot/kernel

Change-Id: Ia2a86d8814d06950ea2d2d19d966c642d26f81d6
Reviewed-on: https://review.haiku-os.org/c/1302
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>