libstdc++: Fix and simplify freestanding configuration [PR103866]

libstdc++: Fix and simplify freestanding configuration [PR103866]

This fixes the --disable-hosted-libstdcxx build so that it works with
--without-headers. Currently you need to also use --with-newlib, which
is confusing for users who aren't actually using newlib.

The AM_PROG_LIBTOOL checks are currently skipped for --with-newlib and
--with-avrlibc builds, with this change they are also skipped when using
--without-headers.  It would be nice if using --disable-hosted-libstdcxx
automatically skipped those checks, but GLIBCXX_ENABLE_HOSTED comes too
late to make the AM_PROG_LIBTOOL checks depend on $is_hosted.

The checks for EOF, SEEK_CUR etc. cause the build to fail if there is no
<stdio.h> available.  Unlike most headers, which get a HAVE_FOO_H macro,
<stdio.h> is in autoconf's default includes, so every check tries to
include it unconditionally. This change skips those checks for
freestanding builds.

Similarly, the checks for <stdint.h> types done by GCC_HEADER_STDINT try
to include <stdio.h> and fail for --without-headers builds. This change
skips the use of GCC_HEADER_STDINT for freestanding. We can probably
stop using GCC_HEADER_STDINT entirely, since only one file uses the
gstdint.h header that is generated, and that could easily be changed to
use <stdint.h> instead. That can wait for stage 1.

We also need to skip the GLIBCXX_CROSSCONFIG stage if --without-headers
was used, since we don't have any of the functions it deals with.

The end result of the changes above is that it should not be necessary
for a --disable-hosted-libstdcxx --without-headers build to also use
--with-newlib.

Finally, compile libsupc++ with -ffreestanding when --without-headers is
used, so that <stdint.h> will use <gcc-stdint.h> instead of expecting it
to come from libc.

libstdc++-v3/ChangeLog:

	PR libstdc++/103866
	* acinclude.m4 (GLIBCXX_COMPUTE_STDIO_INTEGER_CONSTANTS): Do
	nothing for freestanding builds.
	(GLIBCXX_ENABLE_HOSTED): Define FREESTANDING_FLAGS.
	* configure.ac: Do not use AC_LIBTOOL_DLOPEN when configured
	with --without-headers.  Do not use GCC_HEADER_STDINT for
	freestanding builds.
	* libsupc++/Makefile.am (HOSTED_CXXFLAGS): Use -ffreestanding
	for freestanding builds.
	* configure: Regenerate.
	* Makefile.in: Regenerate.
	* doc/Makefile.in: Regenerate.
	* include/Makefile.in: Regenerate.
	* libsupc++/Makefile.in: Regenerate.
	* po/Makefile.in: Regenerate.
	* python/Makefile.in: Regenerate.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.
	* src/filesystem/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.