PE32+ executable : .text segment encryption

If you're going to use assembly stubs, you can just patch it directly to the binary. You don't really need to use c++ to do that for you.
To do that for both 32 and 64 bit, you can use CreateFile, CreateFileMapping, and MapViewOfFile. This will allow you to dump the .text section, basically all sections, of the PE and you can do whatever you want to it. Then you can add a function to decrypt it or whatever and/or write a stub to do the decrypting portion of it in a space big enough to fit that stub or just add another section to the PE

You don't need the assembly language to add a decoder stub for the .text section. You can just write the code in C/C++ compile it as a Position independent executable & extract the ONLY .text section it'll have. After that you can just use a normal PE editor (CFF explorer is pretty robust) to manually add the .stub section and paste the content of the .text section extracted from your position independent compiled code. If you don't wanna go into trouble of creating Position independent code you can just use Position Independent Shellcode available on github.