PR25569, PDP11 ld -s clobbers last data byte

PR25569, PDP11 ld -s clobbers last data byte

This patch fixes an ancient wart in aout support, in that text and
data section sizes are rounded up for alignment rather that just the
corresponding header sizes.  Changing section sizes could conceivably
result in buffer overflows if section contents were held in memory.
Also, keeping the original section sizes allows this PR to be fixed
nicely.

bfd/
	PR 25569
	* aoutx.h (adjust_o_magic, adjust_z_magic, adjust_n_magic): Use
	"text", "data" and "bss" section pointer vars.  Don't update
	section size, just exec header sizes.
	(adjust_sizes_and_vmas): Don't update text section size.  Set
	initial exec header a_text.  Print exec headers sizes.
	* pdp11.c (adjust_o_magic, adjust_z_magic, adjust_n_magic),
	(adjust_sizes_and_vmas): Similarly.  Formatting.
	(final_link): Correct final file extension.
gas/
	PR 25569
	* config/obj-aout.c (obj_aout_frob_file_before_fix): Don't loop
	on section size adjustment, instead perform another write if
	exec header size is larger than section size.