[Ada] Preserve casing of output files

[Pierre-Marie de Rodat]

Windows is case insensitive but also case preserving, so we don't want to
generate a file in lower case if the input file wasn't.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* osint-c.adb (Set_File_Name): Preserve casing of file.
	* osint.adb (File_Names_Equal): New.
	(Executable_Name): Use File_Equal instead of
	Canonical_Case_File_Name.
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