At least as far as major C++ implementations go you pretty much already have a freestanding stdlib since the standard-specified freestanding parts are a subset

At least as far as major C++ implementations go you pretty much already have a freestanding stdlib since the standard-specified freestanding parts are a subset of the hosted parts. It's just a matter of compiling in freestanding "mode" (e.g., passing -ffreestanding to GCC/Clang)