libstdc++: Pass error handler to libbacktrace functions

libstdc++: Pass error handler to libbacktrace functions

Also pass threaded=1 to __glibcxx_backtrace_create_state and remove some
of the namespace scope declarations in the header.

Co-authored-by: François Dumont <frs.dumont@gmail.com>
libstdc++-v3/ChangeLog:

	* include/debug/formatter.h [_GLIBCXX_DEBUG_BACKTRACE]
	(_Error_formatter::_Error_formatter): Pass error handler to
	__glibcxx_backtrace_create_state. Pass 1 for threaded argument.
	(_Error_formatter::_S_err): Define empty function.
	* src/c++11/debug.cc (_Error_formatter::_M_error): Pass error
	handler to __glibcxx_backtrace_full.