binutils: backport R_XTENSA_DIFF* relocation signedness fix

This fixes the following build errors:

  qlatincodec.o:(.debug_line+0xcd7): dangerous relocation: overflow after relaxation
  qmetaobject.o:(.debug_frame+0xa89): dangerous relocation: overflow after relaxation

Fixes:
  http://autobuild.buildroot.net/results/f51/f51c87ae3c883cc36b06c0741999b52e48e4c943/
  http://autobuild.buildroot.net/results/bcb/bcbf25e4054911a7a192bdb5c92e0ebd50e9666c/
  http://autobuild.buildroot.net/results/8a8/8a8a0f8cca293ce1f383421ba8b1223b4266e3bc/
  http://autobuild.buildroot.net/results/a54/a54f3707781dc193292eda7d1723fce6487d6980/



Backported from: 1058c7532d0b012ac329219264ddad59049fb6e6
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>