Genode: add scheduleCallbacks to asyncdispatch (#20708)

* Genode: add native signal handler

* Genode: add scheduleCallbacks to asyncdispatch

This resolves some awkwardness where an RPC server may or may not
use callSoon while dispatching RPC but without scheduling timers
or I/O.

lib/genode/entrypoints.nim

@@ -0,0 +1,22 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## See `Genode Foundations - Entrypoint <https://genode.org/documentation/genode-foundations/21.05/functional_specification/Entrypoint.html>`
+## for a description of Entrypoints.
+
+type
+  EntrypointObj {.
+    importcpp: "Genode::Entrypoint",
+    header: "<base/entrypoint.h>",
+    pure.} = object
+  Entrypoint* = ptr EntrypointObj
+    ## Opaque Entrypoint object.
+
+proc ep*(env: GenodeEnv): Entrypoint {.importcpp: "(&#->ep())".}
+  ## Access the entrypoint associated with `env`.