Fix backtrace generation on Android/ARM/QNX

Fix backtrace generation on Android/ARM/QNX

Use a GCC extension to the common C++ ABI's called _Unwind_Backtrace, which
generate backtraces if possible. Its direct use will ensure we use the
statically linked unwind, which knows about our unwind tables. This also helps
for the setup when libc doesn't have the glibc specific backtrace() function we
used previously. The unwinder on Mac OS X with clang also implements the same
extension.

Change-Id: I0b780590c10c16e50ec570f7da1efae2e64c46dd
Reviewed-by: Lars Knoll <lars.knoll@digia.com>