Now that we require a minimum of MPFR 3.1.0+ to build GCC, we can do
some modernization of the MPFR usage in the GFortran frontend.

This patch replaces

1) GMP_RND* with MPFR_RND*

2) mp_exp_t with mpfr_exp_t

3) mp_prec_t with mpfr_prec_t

4) mp_rnd_t with mpfr_rnd_t

gcc/fortran/ChangeLog:

2019-11-20  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/92463
	* arith.c (gfc_mpfr_to_mpz): Change mp_exp_t to mpfr_exp_t.
	(gfc_check_real_range): Likewise.
	* gfortran.h (GFC_RND_MODE): Change GMP_RNDN to MPFR_RNDN.
	* module.c (mio_gmp_real): Change mp_exp_t to mpfr_exp_t.
	* simplify.c (degrees_f): Change mp_rnd_t to mpfr_rnd_t.
	(radians_f): Likewise.
	(fullprec_erfc_scaled): Change mp_prec_t to mpfr_prec_t.
	(asympt_erfc_scaled): Likewise.
	(gfc_simplify_nearest): Change mp_exp_t to mpfr_exp_t, and
	GMP_RND* to MPFR_RND*.

From-SVN: r278523