Initial port of Libunwind to Unikraft

Initial port of Libunwind to Unikraft

This is our initial port of libunwind to Unikraft as an external
library. Libc, or an equivalent, is required for it to work.

The dependency list in the  Makefile should be:
...:$(UK_LIBS)/libunwind:$(UK_LIBS)/compiler-rt:
$(UK_LIBS)/libcxxabi:$(UK_LIBS)/libcxx:$(UK_LIBS)/newlib:...

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