ada: Allow file mapping for System's spec

ada: Allow file mapping for System's spec

Before this patch, it was never allowed to use pragma Source_File_Name
for the spec of System, allegedly because Targparm.Get_Target_Parameters
is called before configuration pragmas are processed. Using a mapping
file was allowed but did not work correctly.

This patch makes mapping files loading happen before the call to
Get_Target_Parameters so mapping file can set the file name of System.
Also, pragma Source_File_Name is allowed if it confirms a mapping that
was previously given in a mapping file, to accommodate GPRbuild that
uses both pragmas and mapping files.

gcc/ada/ChangeLog:

	* frontend.adb (Frontend): Move call to Fmap.Initialize ...
	* gnat1drv.adb (Gnat1drv): ... here. Look up Fmap when loading System.
	* par-prag.adb (Prag): Allow pragma Source_File_Name for System when
	it confirms an existing mapping.