Fortran: add IEEE_MODES_TYPE, IEEE_GET_MODES and IEEE_SET_MODES

The IEEE_MODES_TYPE type and the two functions that get and set it
were added in Fortran 2018.  They can be implemented using the already
existing target-specific functions.  A future optimization could, on
some targets, set/get all modes through one or two instructions only,
but that would need a new set of functions in all config/fpu-* files.

2022-09-04  Francois-Xavier Coudert  <fxcoudert@gcc.gnu.org>

libgfortran/

	* ieee/ieee_exceptions.F90: Add IEEE_MODES_TYPE, IEEE_GET_MODES
	and IEEE_SET_MODES.
	* ieee/ieee_arithmetic.F90: Make them public in IEEE_ARITHMETIC
	as well.

gcc/testsuite/

	* gfortran.dg/ieee/modes_1.f90: New test.