[Ada] Fix illegal Ada in s-dwalin.adb

[Ada] Fix illegal Ada in s-dwalin.adb

Both the `System.Mmap` and `System.Object_Reader` packages are defining
entities named `Offset` and they are both `use`d at the top of
s-dwalin.adb.

Therefore, the references to `Offset` throughout this file are
ambiguous, and GNAT is supposed to complain. Since it does not for the
moment, we fix the ambiguity by declaring a subtype `Offset` at the top
of the file simply renames `System.Object_Reader.Offset`.

gcc/ada/

	* libgnat/s-dwalin.adb: Add a subtype declaration to fix the
	ambiguity.