Start of relro segment adjustment

Start of relro segment adjustment

Adjusting the start of the relro segment in order to make it end
exactly on a page boundary runs into difficulties when sections in the
relro segment are aligned;  Adjusting the start by (next_page - end)
sometimes results in more than that adjustment occurring at the end,
overrunning the page boundary.  So when that occurs we try a new lower
start position by masking the adjusted start with the maximum section
alignment.  However, we didn't consider that this masked start address
may in fact be before the initial relro base, which is silly since
that can only increase padding at the relro end.

I've also moved some calculations closer to where they are used, and
comments closer to the relevant statements.

	* ldlang.c (lang_size_sections): When alignment of sections
	results in relro base adjustment being too large, don't go lower
	than the initial value.
	* ldexp.c (fold_binary <DATA_SEGMENT_RELRO_END>): Comment.
	* scripttempl/elf.sc (DATA_SEGMENT_ALIGN): Omit SEGMENT_SIZE
	alignment when SEGMENT_SIZE is the same as MAXPAGESIZE.