Target FP: Introduce target-float.{c,h}

This patch introduces the new set of target floating-point handling routines
in target-float.{c,h}.  In the end, the intention is that this file will
contain support for all operations in target FP format, fully replacing
both the current doublest.{c,h} and dfp.{c,h}.

To begin with, this patch only adds a target_float_is_zero routine,
which handles the equivalent of decimal_is_zero for both binary and
decimal FP.  For the binary case, to avoid conversion to DOUBLEST,
this is implemented using the floatformat_classify routine.

However, it turns out that floatformat_classify actually has a bug
(it was not used to check for zero before), so this is fixed as well.

The new routine is used in both value_logical_not and valpy_nonzero.

There is one extra twist: the code previously used value_as_double
to convert to DOUBLEST and then compare against zero.  That routine
performs an extra task: it detects invalid floating-point values
and raises an error.  In any place where value_as_double is removed
in favor of some target-float.c routine, we need to replace that check.

To keep this check centralized in one place, I've added a new routine
is_floating_value, which returns a boolean determining whether a
value's type is floating point (binary or decimal), and if so, also
performs the validity check.  Since we need to check whether a value
is FP before calling any of the target-float routines anyway, this
seems a good place to add the check without much code size overhead.

In some places where we only want to check for floating-point types
and not perform a validity check (e.g. for the *output* of an operation),
we can use the new is_floating_type routine (in gdbarch) instead.

The validity check itself is done by a new target_float_is_valid
routine in target-float, encapsulating floatformat_is_valid.

ChangeLog:
2017-11-06  Ulrich Weigand  <uweigand@de.ibm.com>

	* Makefile.c (SFILES): Add target-float.c.
	(HFILES_NO_SRCDIR): Add target-float.h.
	(COMMON_OBS): Add target-float.o.
	* target-float.h: New file.
	* target-float.c: New file.

	* doublest.c (floatformat_classify): Fix detection of float_zero.

	* gdbtypes.c (is_floating_type): New function.
	* gdbtypes.h (is_floating_type): Add prototype.

	* value.c: Do not include "floatformat.h".
	(unpack_double): Use target_float_is_valid.
	(is_floating_value): New function.
	* value.h (is_floating_value): Add prototype-

	* valarith.c: Include "target-float.h".
	(value_logical_not): Use target_float_is_zero.

	* python/py-value.c: Include "target-float.h".
	(valpy_nonzero): Use target_float_is_zero.

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