Don't produce relocations for a difference in a section with no symbols.

We were producing a relocation for
----------------
.section foo,bar
La:
Lb:
 .long   La-Lb
--------------

but not for

---------------------
  .section foo,bar
zed:
La:
Lb:
 .long   La-Lb
----------------

This patch handles the case where both fragments are part of the first atom
in a section and there is no corresponding symbol to that atom.

This fixes pr21328.

llvm-svn: 221304