Initial port of pthreadpool to Unikraft

Initial port of pthreadpool to Unikraft

This is our initial port of pthreadpool to Unikraft as an external library. Libfxdiv
and pthreads are required. When adding the library to the dependency list, this
library should come after libfxdiv.
(e.g. ...:$(UK_LIBS)/libunwind:$(UK_LIBS)/compiler-rt:$(UK_LIBS)/libcxxabi:
$(UK_LIBS)/libcxx:$(UK_LIBS)/pthread-embedded:$(UK_LIBS)/newlib:
$(UK_LIBS)/fxdiv:(UK_LIBS)/pthreadpool:...)

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