PR middle-end/36757

	* builtins.c (expand_builtin_signbit): Add asserts to make sure
	we can expand BUILT_IN_SIGNBIT inline.
	* builtins.def (BUILT_IN_SIGNBIT): Make type-generic.
	* doc/extend.texi: Document the type-generic __builtin_signbit.

	* c-common.c (check_builtin_function_arguments): Add check
	for BUILT_IN_SIGNBIT argument.

	* gcc.dg/builtins-error.c: Add checks for __builtin_signbit.
	* gcc.dg/tg-tests.h: Add checks for __builtin_signbit.

From-SVN: r226990