fortran: Replace uses of {FLOAT,{,LONG_}DOUBLE}_TYPE_SIZE

Joseph pointed out "floating types should have their mode,
not a poorly defined precision value" in the discussion[1],
as he and Richi suggested, the existing macros
{FLOAT,{,LONG_}DOUBLE}_TYPE_SIZE will be replaced with a
hook mode_for_floating_type.  To be prepared for that, this
patch is to replace use of {FLOAT,{,LONG_}DOUBLE}_TYPE_SIZE
in fortran with TYPE_PRECISION of
{float,{,long_}double}_type_node.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2024-May/651209.html

gcc/fortran/ChangeLog:

	* trans-intrinsic.cc (build_round_expr): Use TYPE_PRECISION of
	long_double_type_node to replace LONG_DOUBLE_TYPE_SIZE.
	* trans-types.cc (gfc_build_real_type): Use TYPE_PRECISION of
	{float,double,long_double}_type_node to replace
	{FLOAT,DOUBLE,LONG_DOUBLE}_TYPE_SIZE.