It isn't integrated into the standard 'C' runtime library, but there is an SD library which can be used to access a filing system on SD cards.
So it isn't ISO compliant, which is kind of the point here.
As I said, the ISO standard has two levels, hosted and freestanding. Freestanding is without libraries, other than what is needed to implement missing instructions (floating point and long long emulation for instance), and the stdarg.h library. So, it could meet a freestanding ISO specification, if you used an appropriate compiler, and could add the options to more strictly conform to the ISO standard.