fix _atomic_sub64 for older clang versions
fix _atomic_sub64 for older clang versions

Praetonus/ponyc,kulibali/ponyc,lukecheeseman/ponyta,Praetonus/ponyc,ponylang/ponyc,jonas-l/ponyc,jemc/ponyc,sgebbie/ponyc,lukecheeseman/ponyta,pap/ponyc,sgebbie/ponyc,Theodus/ponyc,sgebbie/ponyc,darach/ponyc,mkfifo/ponyc,CausalityLtd/ponyc,dckc/ponyc,Theodus/ponyc,boemmels/ponyc,ryanai3/ponyc,kulibali/ponyc,sgebbie/ponyc,doublec/ponyc,Perelandric/ponyc,mkfifo/ponyc,jupvfranco/ponyc,jupvfranco/ponyc,Theodus/ponyc,CausalityLtd/ponyc,jemc/ponyc,Praetonus/ponyc,ponylang/ponyc,boemmels/ponyc,jemc/ponyc,shlomif/ponyc,Praetonus/ponyc,darach/ponyc,dckc/ponyc,ryanai3/ponyc,mkfifo/ponyc,Perelandric/ponyc,dipinhora/ponyc,boemmels/ponyc,jupvfranco/ponyc,jupvfranco/ponyc,dipinhora/ponyc,malthe/ponyc,malthe/ponyc,Theodus/ponyc,ponylang/ponyc,malthe/ponyc,jonas-l/ponyc,kulibali/ponyc,Theodus/ponyc,pap/ponyc,sgebbie/ponyc,mkfifo/ponyc,lukecheeseman/ponyta,boemmels/ponyc,darach/ponyc,CausalityLtd/ponyc,doublec/ponyc,cquinn/ponyc,dipinhora/ponyc,kulibali/ponyc,ryanai3/ponyc,Perelandric/ponyc,shlomif/ponyc,cquinn/ponyc,malthe/ponyc,mkfifo/ponyc,pap/ponyc,boemmels/ponyc,jupvfranco/ponyc,doublec/ponyc,cquinn/ponyc,Perelandric/ponyc,jonas-l/ponyc,cquinn/ponyc,Perelandric/ponyc