Reenable tests and benchmarks for machines, rcu

These were blocked some time ago due to not compiling against GHC
8.6, but as far as I can tell, this is no longer an issue.

build-constraints.yaml

@@ -6416,7 +6416,6 @@ skipped-tests:
     - quickcheck-state-machine
     - rakuten
     - rank1dynamic
-    - rcu
     - rss-conduit
     - servant-auth-server
     - servant-blaze
@@ -7003,7 +7002,6 @@ skipped-benchmarks:
     - lifted-async
     - lifted-base
     - loop
-    - machines
     - matrices
     - matrix
     - megaparsec
@@ -7019,7 +7017,6 @@ skipped-benchmarks:
     - prettyprinter
     - prometheus-client
     - ramus
-    - rcu
     - reinterpret-cast
     - rng-utils
     - sampling