libstdc++: [_GLIBCXX_DEBUG] Add backtrace generation on demand

  Add _GLIBCXX_DEBUG_BACKTRACE macro to activate backtrace generation on
_GLIBCXX_DEBUG assertions. Prerequisite is to have configure the lib with:

--enable-libstdcxx-backtrace=yes

libstdc++-v3/ChangeLog:

	* include/debug/formatter.h
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_state): Declare.
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_create_state): Declare.
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_callback): Define.
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_error_callback): Define.
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full_func): Define.
	[_GLIBCXX_HAVE_STACKTRACE](__glibcxx_backtrace_full): Declare.
	[_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_state): New.
	[_GLIBCXX_HAVE_STACKTRACE](_Error_formatter::_M_backtrace_full): New.
	* src/c++11/debug.cc [_GLIBCXX_HAVE_STACKTRACE](print_backtrace): New.
	(_Error_formatter::_M_error()): Adapt.
	* src/libbacktrace/Makefile.am: Add backtrace.c.
	* src/libbacktrace/Makefile.in: Regenerate.
	* src/libbacktrace/backtrace-rename.h (backtrace_full): New.
	* testsuite/23_containers/vector/debug/assign4_backtrace_neg.cc: New test.
	* doc/xml/manual/debug_mode.xml: Document _GLIBCXX_DEBUG_BACKTRACE.
	* doc/xml/manual/using.xml: Likewise.