builtins: Add __builtin_nextafterf16b builtin

builtins: Add __builtin_nextafterf16b builtin

The following patch adds another needed builtin.
The earlier patch adds among other things __builtin_nextafterf16
builtin which we need in order to constexpr evaluate
std::nextafter(_Float16) overload (patch for that has been posted already).
While there is inline implementation of the overload, it isn't constant
evaluation friendly, and the builtin doesn't need libm implementation
because it will be used only during constant expression evaluation.
We need the same thing also for std::nextafter(__gnu_cxx::__bfloat16_t)
though and this patch does that.

2022-10-31  Jakub Jelinek  <jakub@redhat.com>

	* builtin-types.def (BT_FN_BFLOAT16_BFLOAT16_BFLOAT16): New.
	* builtins.def (BUILT_IN_NEXTAFTERF16B): New builtin.
	* fold-const-call.cc (fold_const_call_sss): Handle
	CFN_BUILT_IN_NEXTAFTERF16B.