ada: Fix Ada.Text_IO.Delete with "encoding=8bits" form

ada: Fix Ada.Text_IO.Delete with "encoding=8bits" form

Before this patch, on Windows, file with non-ASCII Latin1 names could be created
with Ada.Text_IO.Create by passing "encoding=8bits" through the Form
parameter and a Latin1-encoded string through the Name parameter,
but calling Ada.Text_IO.Delete on them raised an illegitimate exception.

This patch fixes this by making the wrappers of the unlink system function
aware of the encoding value passed through the Form parameter. It also
removes an unnecessary curly-brace block.

gcc/ada/

	* adaint.c (__gnat_unlink): Add new parameter and fix text
	conversion on Windows. Remove unnecessary curly braces.
	* adaint.h (__gnat_unlink): Add new parameter.
	* libgnat/i-cstrea.ads (unlink): Adapt to __gnat_unlink signature
	change.
	* libgnat/i-cstrea.adb (unlink): New Subprogram definition.
	* libgnat/s-crtl.ads (unlink): Adapt to __gnat_unlink signature
	change.
	* libgnat/s-fileio.adb (Delete): Pass encoding argument to unlink.