Add the <stacktrace> header and a new libstdc++_libbacktrace.a library
that provides the implementation. For now, the new library is only built
if --enable-libstdcxx-backtrace=yes is used. As with the Filesystem TS,
the new library is only provided as a static archive.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_BACKTRACE): New macro.
	* configure.ac: Use GLIBCXX_ENABLE_BACKTRACE.
	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/std/stacktrace: New header.
	* include/std/version (__cpp_lib_stacktrace): Define.
	* Makefile.in: Regenerate.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* doc/Makefile.in: Regenerate.
	* libsupc++/Makefile.in: Regenerate.
	* po/Makefile.in: Regenerate.
	* python/Makefile.in: Regenerate.
	* src/Makefile.am: Regenerate.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.
	* src/filesystem/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* src/libbacktrace/Makefile.am: New file.
	* src/libbacktrace/Makefile.in: New file.
	* src/libbacktrace/backtrace-rename.h: New file.
	* src/libbacktrace/backtrace-supported.h.in: New file.
	* src/libbacktrace/config.h.in: New file.
	* testsuite/lib/libstdc++.exp (check_effective_target_stacktrace):
	New proc.
	* testsuite/20_util/stacktrace/entry.cc: New test.
	* testsuite/20_util/stacktrace/synopsis.cc: New test.
	* testsuite/20_util/stacktrace/version.cc: New test.