libstdc++: Use ENOSYS for unsupported filesystem ops on AVR

Because avr-libc <errno.h> defines most error numbers with duplicate
values it's not sufficient to check #ifdef ENOTSUP when deciding which
std::errc constant to use for the filesystem library's __unsupported()
helper. Add a special case for AVR to always use the ENOSYS value.

libstdc++-v3/ChangeLog:

	* src/filesystem/ops-common.h [AVR] (__unsupported): Always use
	errc::function_not_supported instead of errc::not_supported.

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

@@ -84,7 +84,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   inline error_code
   __unsupported() noexcept
   {
-#if defined ENOTSUP
+#if defined __AVR__
+    // avr-libc defines ENOTSUP and EOPNOTSUPP but with nonsense values.
+    // ENOSYS is defined though, so use an error_code corresponding to that.
+    // This contradicts the comment above, but we don't have much choice.
+    return std::make_error_code(std::errc::function_not_supported);
+#elif defined ENOTSUP
     return std::make_error_code(std::errc::not_supported);
 #elif defined EOPNOTSUPP
     // This is supposed to be for socket operations