Target FP: Handle interfaces to scripting languages

Target FP: Handle interfaces to scripting languages

The last remaing use for DOUBLEST is in the code that interfaces to the
scripting languages (Python and Guile).  The problem here is that we
expose interfaces to convert a GDB value to and from native values of
floating-point type in those languages, and those by definition use
the host floating-point format.

While we cannot completely eliminate conversions to/from the host
floating-point format here, we still need to get rid of the uses
of value_as_double / value_from_double, since those will go away.

This patch implements two new target-float.c routine:
 - target_float_to_host_double
 - target_float_from_host_double
which convert to/from a host "double".  Those should only ever be
used where a host "double" is mandated by an external interface.

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

	* target-float.c (floatformat_to_host_double): New function.
	(floatformat_from_host_double): Likewise.
	(target_float_to_host_double): Likewise.
	(target_float_from_host_double): Likewise.
	* target-float.h (target_float_to_host_double): Add prototype.
	(target_float_from_host_double): Likewise.

	* guile/scm-value.c: Include "target-float.h".
	(gdbscm_value_to_real): Use target_float_to_host_double.
	Handle integer source values via value_as_long.
	* guile/scm-math.c: Include "target-float.h".  Do not include
	"doublest.h", "dfp.h", and "expression.h".
	(vlscm_convert_typed_number): Use target_float_from_host_double.
	(vlscm_convert_number): Likewise.

	* python/py-value.c (valpy_float): Use target_float_to_host_double.
	(convert_value_from_python): Use target_float_from_host_double.