[Ada] fix support for creating shared libs under darwin

[Arnaud Charlet <charlet at adacore dot com>]

Tested under ppc-darwin, committed on mainline.

On MacOS (Darwin), the linker does not support the -fini switch,
and the compiler add an extra '_' character in the symbols for
exported subprograms.
This fix allows the creation of shared libraries in library projects
on MacOS. The test is to build a shared library using a library project
file: it should now build without error.

2005-03-17  Vincent Celier  <celier@adacore.com>
	    Nicolas Setton  <setton@adacore.com>

	* mlib-tgt-darwin.adb (Build_Dynamic_Library): Remove the "-fini"
	switch, not supported by the linker on Darwin. Add '_' before
	<library>init, as this character is added unconditionally by the
	compiler.
	(Is_Archive_Ext): Replace the wrong library extension ".dyld" by the
	correct one ".dylib". This fixes detection of the archive files when
	building library projects.

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