libstdc++: Support libc with stdio-only I/O in libstdc++

The current libstdc++ basic_file_stdio.cc code assumes a POSIX API
underneath the stdio implementation provided by the host libc. This
means that the host must provide a fairly broad POSIX file API,
including read, write, open, close, lseek and ioctl.

This patch changes basic_file_stdio.cc to only use basic ANSI-C stdio
functions, allowing it to be used with libc implementations like
picolibc which may not have a POSIX operating system underneath.

This is enabled by a new --enable-cstdio=stdio_pure configure option.

Aided-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_CSTDIO): Allow "stdio_pure"
	option and define _GLIBCXX_USE_PURE_STDIO when it is used. Also
	add "stdio_posix" option as an alias for "stdio".
	* config/io/basic_file_stdio.cc [_GLIBCXX_USE_PURE_STDIO]: Only
	use defined stdio entry points for all I/O operations, without
	direct calls to underlying POSIX functions.
	* config.h.in: Regenerate.
	* configure: Regenerate.