Extend mathfn_built_in to handle combined_fn

Extend mathfn_built_in to handle combined_fn

This patch extends mathfn_built_in to handle combined_fn, but keeps the
old built_in_function interface around since it's a common case.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* builtins.h (mathfn_built_in): Add a variant that takes
	a combined_fn.
	* builtins.c: Include case-cfn-macros.h.
	(CASE_MATHFN): Use CASE_CFN_*.
	(CASE_MATHFN_REENT): Use CFN_ codes.
	(mathfn_built_in_2, mathfn_built_in_1): Replace built_in_function
	argument with a combined_fn.
	(mathfn_built_in): Add a variant that takes a combined_fn.
	(expand_builtin_int_roundingfn_2): Update callers accordingly.
	(fold_builtin_sincos, fold_builtin_classify): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@230483 138bc75d-0d04-0410-961f-82ee72b054a4