Initial port of fxdiv to Unikraft

This is our initial port of fxdiv to Unikraft as an external library. Libcxx is
required. When adding the library to the dependency list it should stay after
libcxx (eg. ...:$(UK_LIBS)/libunwind:$(UK_LIBS)/compiler-rt:$(UK_LIBS)/libcxxabi:
$(UK_LIBS)/libcxx:$(UK_LIBS)/newlib:$(UK_LIBS)/fxdiv:...)

Signed-off-by: Vlad-Andrei Badoiu <vlad_andrei.badoiu@stud.acs.upb.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>

Add documentation

Signed-off-by: Vlad-Andrei Badoiu <vlad_andrei.badoiu@stud.acs.upb.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>

Add .gitignore

Signed-off-by: Vlad-Andrei Badoiu <vlad_andrei.badoiu@stud.acs.upb.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>

Initial commit (empty)