Support __builtin_isinf_sign for new floating-point types (PR middle-end/77269).

Support __builtin_isinf_sign for new floating-point types (PR middle-end/77269).

The __builtin_isinf_sign folding uses a type-specific signbit built-in
function, meaning it only works for the types float, double and long
double, not for types such as _FloatN, _FloatNx, __float128.  Since
the signbit built-in function is now type-generic, that can be used
unconditionally, much as the code uses the type-generic isinf built-in
function unconditionally, and this patch makes it do so, thereby
enabling __builtin_isinf_sign (which glibc uses to expand the isinf
macro since that macro in glibc traditionally provided the stronger
guarantees about the return value given by __builtin_isinf_sign) to
work for all floating-point types.

The test gcc.dg/torture/builtin-isinf_sign-1.c needs updating because
it tests that comparisons of calls to __builtin_isinf_sign to
conditional expressions involving __builtin_isinf and
__builtin_signbit* get optimized away, and with a change of what
particular built-in function for signbit is used, GCC doesn't notice
the expressions with type-generic and non-type-generic built-in
functions are equivalent at -O0 or -O1 (it does optimize away the
original test at -O2).

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

	PR middle-end/77269
gcc:
	* builtins.c (fold_builtin_classify): Use builtin_decl_explicit
	(BUILT_IN_SIGNBIT) to expand __builtin_isinf_sign.

gcc/testsuite:
	* gcc.dg/torture/builtin-isinf_sign-1.c: Use __builtin_signbit not
	__builtin_signbitf and __builtin_signbitl in expected generic
	expansion.
	* gcc.dg/torture/float128-tg-2.c, gcc.dg/torture/float128x-tg-2.c,
	gcc.dg/torture/float16-tg-2.c, gcc.dg/torture/float32-tg-2.c,
	gcc.dg/torture/float32x-tg-2.c, gcc.dg/torture/float64-tg-2.c,
	gcc.dg/torture/float64x-tg-2.c, gcc.dg/torture/floatn-tg-2.h: New
	tests.

From-SVN: r239665