libubacktrace: Provide uClibc with backtrace functions

libubacktrace: Provide uClibc with backtrace functions

A new shared object, libubacktrace.so.0 is added to uClibc
to provide backtrace functions to support application self-debugging.
This set of functions requires to dynamically load libgcc_s.so so they
need to call dlopen/dlsym that are provided by libdl. For this reason
they cannot be included into libc.so.0 but are provided by a new library.

User application that wants to use backtrace needs to be compiled with
-fexceptions option and -rdynamic to get full symbols printed and must be
linked against libubacktrace.so

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>