ada: Normalize span generation on different platforms

The total number of characters on a source code line
is different on Windows and Linux based systems
(CRLF vs LF endings). Use the last non line change
character to adjust printing the spans that go over
the end of line.

gcc/ada/

	* diagnostics-pretty_emitter.adb (Get_Last_Line_Char): New. Get
	the last non line change character. Write_Span_Labels use the
	adjusted line end pointer to calculate the length of the span.