Fortran: add Fortran 2018 IEEE_{MIN,MAX} functions

libgfortran/

	* ieee/ieee_arithmetic.F90: Add IEEE_MIN_NUM, IEEE_MAX_NUM,
	IEEE_MIN_NUM_MAG, and IEEE_MAX_NUM_MAG functions.

gcc/fortran/

	* f95-lang.cc (gfc_init_builtin_functions): Add fmax() and
	fmin() built-ins, and their variants.
	* mathbuiltins.def: Add FMAX and FMIN built-ins.
	* trans-intrinsic.cc (conv_intrinsic_ieee_minmax): New function.
	(gfc_conv_ieee_arithmetic_function): Handle IEEE_MIN_NUM and
	IEEE_MAX_NUM functions.

gcc/testsuite/
	* gfortran.dg/ieee/minmax_1.f90: New test.