Darwin : Avoid a C++ ODR violation seen with LTO.

We have a similar code pattern in darwin-c.c to one in c-pragmas
(most likely a cut & paste) with a struct type used locally to the
TU.  With C++ we need to rename the type to avoid an ODR violation.

gcc/ChangeLog:

	* config/darwin-c.c (struct f_align_stack): Rename
	to type from align_stack to f_align_stack.
	(push_field_alignment): Likewise.
	(pop_field_alignment): Likewise.