Fortran: procedures with BIND(C) attribute require explicit interface [PR85877]

gcc/fortran/ChangeLog:

	PR fortran/85877
	* resolve.cc (resolve_fl_procedure): Check for an explicit interface
	of procedures with the BIND(C) attribute (F2018:15.4.2.2).

gcc/testsuite/ChangeLog:

	PR fortran/85877
	* gfortran.dg/pr85877.f90: New test.