libstdc++: Fix build failures for avr

The abr-libc <errno.h> does not define EOVERFLOW, which means that
std::errc::value_too_large is not defined, and so <charconv> cannot be
compiled. Define value_too_large for avr with a value that does not
clash with any that is defined in <errno.h>. This is a kluge to fix
bootstrap for avr; it can be removed after PR libstdc++/104883 is
resolved.

The avr-libc <errno.h> fails to meet the C and POSIX requirements that
each error macro has a distinct integral value, and is usable in #if
directives. Add a special case for avr to system_error.cc so that only
the valid errors are recognized. Also disable the errno checks in
std::filesystem::remove_all that assume a meaningful value for errno.

On avr-libc <unistd.h> exists but does not define the POSIX functions
needed by std::filesystem, so _GLIBCXX_HAVE_UNISTD_H is not sufficient
to check for basic POSIX APIs. Check !defined __AVR__ as well as
_GLIBCXX_HAVE_UNISTD_H before using those functions. This is a kluge and
we should really have a specific macro that says the required functions
are available.

libstdc++-v3/ChangeLog:

	* config/os/generic/error_constants.h (errc::value_too_large)
	[__AVR__]: Define.
	* src/c++11/system_error.cc
	(system_category::default_error_condition) [__AVR__]: Only match
	recognize values equal to EDOM, ERANGE, ENOSYS and EINTR.
	* src/c++17/fs_ops.cc (fs::current_path) [__AVR__]: Do not check
	for ENOENT etc. in switch.
	(fs::remove_all) [__AVR__]: Likewise.
	* src/filesystem/ops-common.h [__AVR__]: Do not use POSIX open,
	close etc.