ARM: kernel: sort relocation sections before allocating PLTs

The PLT allocation routines try to establish an upper bound on the
number of PLT entries that will be required at relocation time, and
optimize this by disregarding duplicates (i.e., PLT entries that will
end up pointing to the same function). This is currently a O(n^2)
algorithm, but we can greatly simplify this by
- sorting the relocation section so that relocations that can use the
  same PLT entry will be listed adjacently,
- disregard jump/call relocations with addends; these are highly unusual,
  for relocations against SHN_UNDEF symbols, and so we can simply allocate
  a PLT entry for each one we encounter, without trying to optimize away
  duplicates.

Tested-by: Jongsung Kim <neidhard.kim@lge.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>