implement os_self_exe_path in the c++ compiler for darwin
implement os_self_exe_path in the c++ compiler for darwin

ported from the zig std lib

this fixes looking for zig std lib at runtime on darwin

Dimenus/zig,andrewrk/zig,Dimenus/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,Dimenus/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig