[ios::failure] p2: "When throwing ios_base::failure exceptions,
implementations should provide values of ec that identify the specific
reason for the failure."

This adds a new overload of __throw_ios_failure that can be passed
errno, to store error_code(errno, system_category()) in the exception
object.

	PR libstdc++/85343
	* acinclude.m4 (libtool_VERSION): Bump version.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Add new symbol version.
	Export new symbol.
	* configure: Regenerate.
	* doc/xml/manual/abi.xml: Document new versions.
	* include/bits/fstream.tcc (basic_filebuf<C, T>::underflow)
	(basic_filebuf<C, T>::xsgetn): Pass errno to __throw_ios_failure.
	* include/bits/functexcept.h (__throw_ios_failure(const char*, int)):
	Declare new overload.
	* src/c++11/cxx11-ios_failure.cc (__ios_failure): Add new constructor
	and static member function.
	(__throw_ios_failure(const char*, int)): Define.
	* src/c++98/ios_failure.cc [!_GLIBCXX_USE_DUAL_ABI]
	(__throw_ios_failure(const char*, int)): Define.
	* testsuite/util/testsuite_abi.cc: Update known and latest versions.

From-SVN: r263535