[Ada] Define versions of dimension system for Float and Long_Float

[Ada] Define versions of dimension system for Float and Long_Float

The dimension system in System.Dim.Mks is based on Long_Long_Float,
which may not be convenient to people who want to use Float or
Long_Float as basis. These new files provide units that define the
dimension system based on Float in System.Dim.Float_Mks and on
Long_Float in System.Dim.Long_Mks.

Child packages Other_Prefixes and Mks_IO are also defined appropriately,
with new instantiations for the various sizes of floating-point types.

2018-08-21  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* doc/gnat_ugn/gnat_and_program_execution.rst: Update
	documentation of dimensionality analysis.
	* gnat_ugn.texi: Regenerate.
	* Makefile.rtl, impunit.adb: Consider the new units.
	* libgnat/s-dfmkio.ads, libgnat/s-dfmopr.ads,
	libgnat/s-diflmk.ads: New units based on Float.
	* libgnat/s-dilomk.ads, libgnat/s-dlmkio.ads,
	libgnat/s-dlmopr.ads: New units based on Long_Float.
	* libgnat/s-dmotpr.ads: Rename to libgnat/s-dgmgop.ads and turn
	into an instance of
	System.Dim.Generic_Mks.Generic_Other_Prefixes.
	* libgnat/s-dimmks.ads: Rename to libgnat/s-digemk.ads and turn
	into an instance of System.Dim.Generic_Mks.

From-SVN: r263715