[Ada] Output.w always writes to stderr

[Ada] Output.w always writes to stderr

There are several debugging procedures called Output.w, and some
output-redirection features. This patch modifies Output.w so their
output is not redirected; it always goes to standard error. Otherwise,
debugging output can get mixed in with some "real" output (perhaps to a
file), which causes confusion and in some cases failure to build.

gcc/ada/

	* output.adb (Pop_Output, Set_Output): Unconditionally flush
	output when switching from one output destination to another.
	Otherwise buffering can cause garbled output.
	(w): Push/pop the current settings, and temporarily
	Set_Standard_Error during these procedures.