libstdc++: Fix --enable-cstdio=stdio_pure [PR110574]

When configured with --enable-cstdio=stdio_pure we need to consistently
use fseek and not mix seeks on the file descriptor with reads and writes
on the FILE stream.

There are also a number of bugs related to error handling and return
values, because fread and fwrite return 0 on error, not -1, and fseek
returns 0 on success, not the file offset.

libstdc++-v3/ChangeLog:

	PR libstdc++/110574
	* acinclude.m4 (GLIBCXX_CHECK_LFS): Check for fseeko and ftello
	and define _GLIBCXX_USE_FSEEKO_FTELLO.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* config/io/basic_file_stdio.cc (xwrite) [_GLIBCXX_USE_STDIO_PURE]:
	Check for fwrite error correctly.
	(__basic_file<char>::xsgetn) [_GLIBCXX_USE_STDIO_PURE]: Check for
	fread error correctly.
	(get_file_offset): New function.
	(__basic_file<char>::seekoff) [_GLIBCXX_USE_STDIO_PURE]: Use
	fseeko if available. Use get_file_offset instead of return value
	of fseek.
	(__basic_file<char>::showmanyc): Use get_file_offset.