PR25125, relaxation chooses wrong branch size

PR25125, relaxation chooses wrong branch size

The patch I made for PR12049 didn't test for a "negative" branch
properly.  "if (target < address)" ought to have been
"if (target < address + fragP->fr_fix)".  Rather than making that
change, this patch adds fragP->fr_fix into address earlier.  The patch
also avoids running into a bad interaction with the m68k
md_prepare_relax_scan by returning zero growth immediately, since the
adjusted target expression would result in a zero "aim".

	PR gas/25125
	PR gas/12049
	* write.c (relax_frag): Correct calculation of delta for
	positive branches where "stretch" would make the branch
	negative.  Return zero immediately in that case.  Correct
	TC_PCREL_ADJUST comment.