libstdc++: Add libstdc++_libbacktrace.a to libstdc++exp

libstdc++: Add libstdc++_libbacktrace.a to libstdc++exp

This completes the fixes to put all experimental symbols into
libstdc++exp.a.

On trunk the libstdc++_libbacktrace.a was removed completely and its
contents aded to libstdc++exp.a instead. We don't want to remove it on
the gcc-13 branch because that would break makefiles using it. We can
add the contents to libstdc++exp.a and then install a symlink so that
using -lstdc++_libbacktrace still works, but links to libstdc++exp.a
instead.

The libstdc++_libbacktrace.la libtool control file is removed by this
change, because I'm pretty sure it's not actually useful, and I don't
know whether it should be a symlink to libstdc++exp.la or a regular file
that refers to libstdc++_libbacktrace.a.

libstdc++-v3/ChangeLog:

	* src/experimental/Makefile.am (install-exec-local): New target.
	(uninstall-local): New target.
	* src/experimental/Makefile.in: Regenerate.
	* src/libbacktrace/Makefile.am: Build libstdc++_libbacktrace as
	noinst_LTLIBRARIES so it's only a convenience library.
	* src/libbacktrace/Makefile.in: Regenerate.