simplified pdcp_benchmark rule for building standalone. Will later also serve for isolated CU-U implementation. minor compilation issue remaining

Develop Integration Branch -- 2020 week 19

The following Merge Requests were included:

* MR [810] : NR PUCCH
* MR [812] : fix 20 compilation warnings
* MR [815] : rlc v2 -- coverity scan fixes
* MR [816] : hotfix: fix compilation of UE with --musim

Also add fixes for CI

The following was failing:
./build_oai --UE --musim

Not sure this fix is correct, but it seems consistent with
the rest of platform_constants.h.

Minor fixes, doesn't change anything. Not sure these are 'bugs' either,
but let's be polite with coverity scan...

One thing was not changed.
Coverity scan says:
*** CID 357991:  Memory - illegal accesses  (USE_AFTER_FREE)
/home/carabe/raphael/openairinterface5g/openair2/LAYER2/rlc_v2/rlc_entity_am.c: 507 in tx_list_remove_sn()
501         } else {
502           prev = cur;
503           cur = cur->next;
504         }
505       }
506
>>>     CID 357991:  Memory - illegal accesses  (USE_AFTER_FREE)
>>>     Using freed pointer "head.next".
507       return head.next;
508     }
509
510     void cleanup_sdu_list(rlc_entity_am_t *entity)
511     {
512       rlc_sdu_t head;

But as far as I understand, there is no problem. We don't access
head.next at all if it has been freed. Or is there some aliasing
going on there (pointer aliasing)? I doubt it. False positive?