This is documented[0]:
> Note that it is assumed that a freestanding environment will additionally provide memcpy, memmove, memset, and memcmp implementations, as these are needed for efficient codegen for many programs.
GCC has effectively the same stipulation[1]:
> GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp.
The main thing I'm confused about is why you didn't get a linker error.
0. https://clang.llvm.org/docs/CommandGuide/clang.html#cmdoptio...