Fix failing filesystem tests on mingw targets

Fix failing filesystem tests on mingw targets

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Add exports for fstream
	constructors and open members taking wide strings. Fix patterns for
	filesystem::path members to match wstring_view parameters. Add
	exports for shared_ptr members used by directory iterators.
	* src/c++17/fs_ops.cc (remove(const path&, error_code&)): Clear the
	error code parameter if the file doesn't exist.
	* src/filesystem/ops.cc (remove(const path&, error_code&)):
	Likewise.
	* testsuite/27_io/filesystem/operations/canonical.cc: Fix expected
	values for mingw targets, where "/" is not an absolute path. Do not
	test symlinks on mingw targets.
	* testsuite/experimental/filesystem/operations/canonical.cc: Likewise.
	* testsuite/27_io/filesystem/operations/copy.cc: Do not test symlinks
	on mingw targets.
	* testsuite/experimental/filesystem/operations/copy.cc: Likewise.
	* testsuite/27_io/filesystem/operations/create_directories.cc: Check
	that each component of the path is created.
	* testsuite/experimental/filesystem/operations/create_directories.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/exists.cc: Do not test
	permissions on mingw targets.
	* testsuite/experimental/filesystem/operations/exists.cc: Likewise.
	* testsuite/27_io/filesystem/operations/is_empty.cc: Likewise.
	* testsuite/experimental/filesystem/operations/is_empty.cc: Likewise.
	* testsuite/27_io/filesystem/operations/permissions.cc: XFAIL for
	mingw targets.
	* testsuite/experimental/filesystem/operations/permissions.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/remove.cc: Do not test
	symlinks or permissions on mingw targets.
	* testsuite/experimental/filesystem/operations/remove.cc: Likewise.
	* testsuite/27_io/filesystem/operations/remove_all.cc: Do not test
	symlinks on mingw targets.
	* testsuite/experimental/filesystem/operations/remove_all.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/status.cc: Do not test
	permissions on mingw targets.
	* testsuite/27_io/filesystem/operations/weakly_canonical.cc: Do not
	test symlinks on mingw targets.
	* testsuite/experimental/filesystem/operations/space.cc: Fix test
	for mingw targets.

From-SVN: r267991