Re-read section 4 of the C Ianguage Standard
No, I have much better things to do with my time than jump through software politically correct hoops deciding if something is or isn't part of a core language. In my book if it needs an include file then it isn't period. Most of C is written in C and has to be added later. It was designed like that and has developed like that. If I was in to compiler writing then yes the standard would be important, but I am not and I have no need to.
There is a myth that C is portable, this was never true, it isn't true and it will never be true. Sure it is quite portable but in any real system you will always have to change something to get it to run on another system, or even the same system but another environment.
Without being personal, as I have never had to work with you, I have had to manage people who show this attitude to standards. They have been all, without exception, very good at the nit picking, and hopeless at actually getting stuff to work correctly in the space allotted, and the speed required, on any given processor.
The simple fact that I/O is at different addresses on different processors means there is always going to be some fettling needed when switching processors. Couple that with different types of built in hardware, UARTS, I2C, SPI, A/D, D/D and other such goodies completely knocks on the head the concept of portability at the level the OP is asking about.
Since that acid test eliminates all standard functions
Standard functions are what they say standard functions, not part of the core language.