ld: Add fold_segment_align/fold_segment_relro_end/fold_segment_end

ld: Add fold_segment_align/fold_segment_relro_end/fold_segment_end

Extract DATA_SEGMENT_END/DATA_SEGMENT_ALIGN/DATA_SEGMENT_RELRO_END cases
for GNU_RELRO segment into separate functions so that they can also be
used for text-only LOAD segment.

	* ldexp.c (fold_unary): Extract the DATA_SEGMENT_END case to ...
	(fold_segment_end): New function.
	(fold_binary): Extract the DATA_SEGMENT_ALIGN case to ...
	(fold_segment_align): New function.
	(fold_binary): Extract the DATA_SEGMENT_RELRO_END case to ...
	(fold_segment_relro_end): New function.