[Ada] Pass -shared-libgcc when building Ada shared libraries on Windows

[Arnaud Charlet <charlet at adacore dot com>]

Unlike on most platforms, the compiler doesn't automatically set -shared-libgcc
when building shared libraries on Windows.  Now a shared libgcc is required for
the propagation of Ada exceptions in this case so we explicitly pass it, as is
done in C++.

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

2010-10-05  Eric Botcazou  <ebotcazou@adacore.com>

	* mlib-tgt-specific-mingw.adb (No_Argument_List): Delete.
	(Shared_Libgcc): New aliased variable.
	(Shared_Libgcc_Switch): New constant.
	(Build_Dynamic_Library): Pass Shared_Libgcc_Switch to the compiler
	* gcc-interface/Makefile.in (gnatlib-shared-win32): Pass -shared-libgcc
	to the compiler.

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