Silence ARI warning about floatformat_to_double

Silence ARI warning about floatformat_to_double

This silences ARI at the one spot that is permitted to call
floatformat_to_double, and also removes the corresponding "fix" call
from gdb_ari.sh -- it was incorrect, and now is not needed.

gdb/ChangeLog
2019-12-13  Tom Tromey  <tromey@adacore.com>

	* contrib/ari/gdb_ari.sh: Remove "fix" call for
	floatformat_to_double.
	* target-float.c (host_float_ops<T>::from_target): Add ARI
	comment.

Change-Id: I778a17a04da417c113194004dd7de3b1df381266