libstdc++: Export basic_stringbuf constructor [PR 97729]

libstdc++-v3/ChangeLog:

	PR libstdc++/97729
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.29): Add exports.
	* src/c++20/sstream-inst.cc (basic_stringbuf): Instantiate
	private constructor taking __xfer_bufptrs.