fix Issue 11251 - Not able to link (Previous Definition Different)

By #2540 , implicitly defined C main has been moved to __entrypoint module. But, the codegen generates ModuleInfo symbol for the module. And vibe.d has its own main function in library. Then, compiling uer progam with vibe.d library will cause the conflict of two ModuleInfo symbols of __entrypoint module.

Due to avoid ModuleInfo symbol generation for __entrypoint, I added a special case in Module::genobjfile function. But, I'm not sure that the change won't cause any other problems...
Merge pull request #2685 from 9rnsr/fix11251

[REG2.064a] Issue 11251 - Not able to link (Previous Definition Different)