libphobos: Fallback on UnwindBacktrace if LibBacktrace not defined.

libphobos: Fallback on UnwindBacktrace if LibBacktrace not defined.

In the gcc.backtrace module, either one of LibBacktrace or
UnwindBacktrace will always be defined.  Giving UnwindBacktrace a higher
precedence over the libc backtrace as the default handler because the
latter depends on a rt.backtrace module that is not compiled in.

libphobos/ChangeLog:

	* libdruntime/core/runtime.d (defaultTraceHandler): Give
	UnwindBacktrace handler precedence over backtrace.

From-SVN: r268836