In the GimpBacktrace Linux backend, always use libunwind, when
available, to find symbol names, even if dladdr() or libbacktrace
had already found one.  libunwind provides more descriptive names
in certain cases, and, in particular, full symbol names for C++
lambdas.

Note that, in some cases, this can result in a discrepancy between
the reported symbol name, and the corresponding source location.

(cherry picked from commit 72fc0174)