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.

libstdc++-v3/config/os/generic/error_constants.h

@@ -167,6 +167,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #ifdef EOVERFLOW
       value_too_large = 			EOVERFLOW,
+#elif defined __AVR__
+      value_too_large = 			999,
 #endif
 
       wrong_protocol_type = 			EPROTOTYPE

libstdc++-v3/src/c++11/system_error.cc

@@ -251,6 +251,15 @@ namespace
 	X (WRITE_PROTECT,		EROFS);
 #undef X
 
+#elif defined __AVR__
+      // avr-libc only defines a few distinct error numbers. Most <errno.h>
+      // constants are not usable in #if directives and have the same value.
+      case EDOM:
+      case ERANGE:
+      case ENOSYS:
+      case EINTR:
+      case 0:
+	return std::error_condition(ev, generic_category_instance.obj);
 #else
       // List of errno macros from [cerrno.syn].
       // C11 only defines EDOM, EILSEQ and ERANGE, the rest are from POSIX.

libstdc++-v3/src/c++17/fs_ops.cc

@@ -736,7 +736,7 @@ fs::path
 fs::current_path(error_code& ec)
 {
   path p;
-#ifdef _GLIBCXX_HAVE_UNISTD_H
+#if defined _GLIBCXX_HAVE_UNISTD_H && ! defined __AVR__
 #if defined __GLIBC__ || defined _GLIBCXX_FILESYSTEM_IS_WINDOWS
   if (char_ptr cwd = char_ptr{posix::getcwd(nullptr, 0)})
     {
@@ -1302,6 +1302,7 @@ fs::remove_all(const path& p)
     }
     // Directory is empty now, will remove it below.
     break;
+#ifndef __AVR__
   case ENOENT:
     // Our work here is done.
     return 0;
@@ -1309,6 +1310,7 @@ fs::remove_all(const path& p)
   case ELOOP:
     // Not a directory, will remove below.
     break;
+#endif
   default:
     // An error occurred.
     _GLIBCXX_THROW_OR_ABORT(filesystem_error("cannot remove all", p, ec));
@@ -1339,6 +1341,7 @@ fs::remove_all(const path& p, error_code& ec)
     }
     // Directory is empty now, will remove it below.
     break;
+#ifndef __AVR__
   case ENOENT:
     // Our work here is done.
     ec.clear();
@@ -1347,6 +1350,7 @@ fs::remove_all(const path& p, error_code& ec)
   case ELOOP:
     // Not a directory, will remove below.
     break;
+#endif
   default:
     // An error occurred.
     return -1;

libstdc++-v3/src/filesystem/ops-common.h

@@ -167,7 +167,7 @@ namespace __gnu_posix
     return ret;
   }
   using char_type = wchar_t;
-#elif defined _GLIBCXX_HAVE_UNISTD_H
+#elif defined _GLIBCXX_HAVE_UNISTD_H && ! defined __AVR__
   using ::open;
   using ::close;
 # ifdef _GLIBCXX_HAVE_SYS_STAT_H