libubacktrace: fix backtrace for statically linked application

libubacktrace: fix backtrace for statically linked application

libgcc_s.so's unwinder could not access unwind tables of statically
linked binaries, so we really want to use _Unwind_* stuff from
libgcc_eh.a.
It required to build backtrace.c differentiating between shared and
static case.

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>