Should use the built-in std head files of GCC.

[ZhUyU1997]

What is -ffreestanding?

https://gcc.gnu.org/onlinedocs/gcc/Standards.html
The ISO C standard defines (in clause 4) two classes of conforming implementation.
A conforming hosted implementation supports the whole standard including all the library facilities;
a conforming freestanding implementation is only required to provide certain library facilities:
those in <float.h>, <limits.h>, <stdarg.h>, and <stddef.h>;
since AMD1, also those in <iso646.h>;
since C99, also those in <stdbool.h> and <stdint.h>;
and since C11, also those in <stdalign.h> and <stdnoreturn.h>.
In addition, complex types, added in C99, are not required for freestanding implementations.