Find some problem about include files

[SegmentationFaultCD]

1. In file stddef.h, I find that you use typedef to create a type alias, but not use using. And there is a function called unreachable in C++23, you don't have to make a define unreachable .
2. The project called limine-xmake-cxx-template, but it seems to suppost C language
3. you should use cstddef to replace stddef.h, use cstdint to replace stdint.h

[SegmentationFaultCD]

What's more, cstddef and cstdint are freestanding header files, you don't have to provide them.

[wenxuanjun]

These header files were copied directly from the C version of the repository and they are supposed to be removed.