ada: Replace use of 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 LONG_DOUBLE_TYPE_SIZE in ada
with TYPE_PRECISION of long_double_type_node.

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

gcc/ada/ChangeLog:

	* gcc-interface/decl.cc (gnat_to_gnu_entity): Use TYPE_PRECISION of
	long_double_type_node to replace LONG_DOUBLE_TYPE_SIZE.