[Ada] Issue with gnatbind -V switch and Ada 2012

[Pierre-Marie de Rodat]

When compiling the binder generated file in Ada 2012 mode and using
the gnatbind -V switch, invalid characters are flagged.

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

gcc/ada/

	* bindgen.adb (Gen_Bind_Env_String): Generate Ada 2012 compatible
	strings. Code cleanup.
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