C+CC: fix regressions CC AV->MAGICAL in 5.6, PMf_ONCE <5.10, ARRAY_utf8<5.10

B::AV->MAGICAL is not defined in 5.6, skip it in B::CC
PMf_ONCE is not needed <5.10
Skip ARRAY_utf8 <5.10, no shared heks there (and different sv class ids)