flambda-backend: Configure flag to use the 5.x runtime; make the 5.x runtime build (#2004)

* Configure flag to use the 5.x runtime; make the 5.x runtime build

* Add flambda2_runtime5 CI job

* Add build_upstream_closure_runtime5 job

* Fix magic number

* Remove --verbose from dune command

* Fix st_stubs.c for exception_pointer rename to match 5.x

* Expect runtime5 CI jobs to fail

* Missed settings of RUNTIME_DIR

* Missed settings of RUNTIME_DIR again

* Progress towards making the make build work

* Continue fixing make build for runtime4

* Expect runtime5 CI jobs to fail - fixing

* Expect runtime5 CI jobs to fail - fixing again

* CI script debugging

Author: mshinwell

.gitignore

@@ -358,3 +358,6 @@ META
 /yacc/.gdb_history
 
 /otherlibs/dynlink/natdynlinkops
+
+dune.runtime_selection
+