backtrace(): Use libgcc_eh.a instead of libunwind.a.

This patch changes backtrace() to use the _Unwind_* facilities provided
by the GCC runtime (libgcc_eh.a), instead of the separate libunwind.a.

After this patch, we don't use libunwind.a in OSv any more, and it can
be removed (see issue #83).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>