libstdc++: Simplify std::vprint_unicode for non-Windows targets

libstdc++: Simplify std::vprint_unicode for non-Windows targets

Since we don't need to do anything special to print Unicode on
non-Windows targets, we might as well just use std::vprint_nonunicode to
implement std::vprint_unicode. Removing the duplicated code should
reduce code size in cases where those calls aren't inlined.

Also use an RAII type for the unused case where a non-Windows target
calls __open_terminal(streambuf*) and needs to fclose the result. This
makes the code futureproof in case we ever start using the
__write_terminal function for non-Windows targets.

libstdc++-v3/ChangeLog:

	* include/std/ostream (vprint_unicode) [_WIN32]: Use RAII guard.
	(vprint_unicode) [!_WIN32]: Just call vprint_nonunicode.
	* include/std/print (vprint_unicode) [!_WIN32]: Likewise.