Darwin: Fix a type mismatch warning for a non-GCC bootstrap compiler.

DECL_MD_FUNCTION_CODE() returns an int, on one particular compiler the
code in darwin_fold_builtin() triggers a warning.

Fixed thus.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	* config/darwin.cc (darwin_fold_builtin): Make fcode an int to
	avoid a mismatch with DECL_MD_FUNCTION_CODE().