libstdc++: Add Filesystem TS and std::stacktrace symbols to libstdc++exp.a

libstdc++: Add Filesystem TS and std::stacktrace symbols to libstdc++exp.a

This consolidates the three static archives for extensions into one, so
that -lstdc++exp can be used to provide the definitions of all unstable
library features.

For this gcc-13 backport libstdc++_libbacktrace.a is still installed,
but its contents are duplicated in libstdc++exp.a. This means that users
can start using -lstdc++exp with GCC 13.3 if they want, but existing
Makefiles can continue to use -lstdc++_libbacktrace for any GCC 13.x
release.

The libstdc++fs.a archive is still installed, but its contents are
duplicated in libstdc++exp.a now. This means -lstdc++exp can be used
instead of -lstdc++fs. For targets using the GNU linker we should
consider replacing libstdc++fs.a with a linker script that does
INPUT(libstdc++exp.a).

The tests for <experimental/filesystem> could be changed to use
-lstdc++exp instead of -lstdc++fs, which would allow removing
src/filesystem/.libs from the LDFLAGS in scripts/testsuite_flags.in,
but that can be done at a later date.

libstdc++-v3/ChangeLog:

	* doc/html/manual/*: Regenerate.
	* doc/xml/manual/using.xml: Update documentation on linking.
	* scripts/testsuite_flags.in: Adjust LDFLAGS to find
	libstdc++exp instead of libstdc++_libbacktrace.
	* src/c++20/Makefile.am: Fix comment.
	* src/c++20/Makefile.in: Regenerate.
	* src/experimental/Makefile.am: Use LIBADD to include other
	libraries.
	* src/experimental/Makefile.in: Regenerate.
	* testsuite/19_diagnostics/stacktrace/current.cc: Adjust
	dg-options to use -lstdc++exp.
	* testsuite/19_diagnostics/stacktrace/entry.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/stacktrace.cc: Likewise.
	* testsuite/23_containers/vector/debug/assign4_backtrace_neg.cc:
	Likewise.

(cherry picked from commit b96b554592c5cbb6a2c1797ffcb5706fd295f4fd)