builtins: Add DFP signaling NaN built-in functions

Add built-in functions __builtin_nansd32, __builtin_nansd64 and
__builtin_nansd128 to return signaling NaNs of decimal floating-point
types, analogous to the functions already present for binary
floating-point types.

This patch, independent of
<https://gcc.gnu.org/pipermail/gcc-patches/2020-October/557136.html>
(pending review), is in preparation for adding the <float.h> macros
for such signaling NaNs that are in C2x, analogous to the macros for
other types that are in that patch.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.  Also ran
the new tests for powerpc64le-linux-gnu to confirm they do work in the
case (hardware DFP) where floating-point exceptions are supported for
DFP.

gcc/
2020-11-06  Joseph Myers  <joseph@codesourcery.com>

	* builtins.def (BUILT_IN_NANSD32, BUILT_IN_NANSD64)
	(BUILT_IN_NANSD128): New built-in functions.
	* fold-const-call.c (fold_const_call): Handle the new built-in
	functions.
	* doc/extend.texi (__builtin_nansd32, __builtin_nansd64)
	(__builtin_nansd128): Document.
	* doc/sourcebuild.texi (Effective-Target Keywords): Document
	fenv_exceptions_dfp.

gcc/testsuite/
2020-11-06  Joseph Myers  <joseph@codesourcery.com>

	* lib/target-supports.exp
	(check_effective_target_fenv_exceptions_dfp): New.
	* gcc.dg/dfp/builtin-snan-1.c, gcc.dg/dfp/builtin-snan-2.c: New
	tests.