Print more information about relocations.

With this patch llvm-readobj now prints if a relocation is pcrel, its length,
if it is extern and if it is scattered.

It also refactors the code a bit to use bit fields instead of shifts and
masks all over the place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179294 91177308-0d34-0410-b5e6-96231b3b80d8