bfd: fix -Wunused errors with clang 13+

Clang 13 and 14 produce some -Wunused-but-set-{variable,parameter} for
situations where gcc doesn't.  In particular, when a variable is set and
then used in a way to update its own value.  For example, if `i` is only
used in this way:

  int i = 2;
  i++;
  i = i + 1;

gcc won't warn, but clang will.

Fix all such errors found in an --enable-targets=all build.  It would be
important for somebody who knows what they're doing to just make sure
that these variables can indeed be deleted, and that there a no cases
where it's a bug, and the variable should actually be used.

The first instance of this error fix by this patch is:

      CC       elf32-score.lo
    /home/simark/src/binutils-gdb/bfd/elf32-score.c:450:11: error: variable 'relocation' set but not used [-Werror,-Wunused-but-set-variable]
      bfd_vma relocation;
              ^

Change-Id: I2f233ce20352645cf388aff3dfa08a651d21a6b6