shared: move ScopeExit from riscv64 arch kernel code

this file is c++11 only.

Change-Id: Ibff7acec00337a9f56f9b8e29ea262c8d64c2446
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5292
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>