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

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.