ada: GNAT UGN: Add section documenting PIE being enabled by default on Linux

ada: GNAT UGN: Add section documenting PIE being enabled by default on Linux

This commit updates the Linux-specific chapter to add a new section
documenting the fact that PIE is enabled by default, and provides
some information about the impact that this might have on some
projects, as well as recommendations on how to handle issues.

gcc/ada/

	* doc/gnat_ugn/platform_specific_information.rst
	(_PIE_Enabled_By_Default_On_Linux): New section.
	* gnat-style.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.