C: unify rv => get_cv logic

Moose::Util::TypeConstraints::Builtins::_RegexpRef with 5.8 or 5.10 returns a PVNV PV as get_cv.
Check for that now in save_pv_or_rv setting static=0
C 5.10+5.12: get rid of seperate struct SVPV

with c99 compilers. use .svu_pv= initialization on c99 compilers,
only use seperate struct SVPV with non c99 cc