ada: Rework GNATdll shared library relocation support.

ada: Rework GNATdll shared library relocation support.

The code has been simplified to use a single way to create a DLL.
The relocation support is based on whether the base address for the
DLL is passed to the final linker step or not.

gcc/ada/ChangeLog:

	* mdll.adb: Use the same procedure to create relocatable or non
	relocatable DLL. The only difference is wether the base address is
	passed to the final linker. If no base-address is given the DLL is
	relocatable.