Just in case somebody will need it...
Libftasm is a project which goal is to reimplement some of the standard libc functions in pure assembly. In this specific case I used x64 Intel assembly and NASM assembler.
To build a static library just run make.
To build some simple tests as well, execute command make test
and run test binary.
The library builds and works well at least on macOS Mojave (10.14)
It also builds on Linux, but ft_is* functions don't work for some reason,
which I've yet to figure out.