I created a base class Rel32Finder from which derives both Rel32FinderX86 and Rel32Finder64.
This makes ParseRel32RelocsFromSection method more similar in x64 and x86.
Note that I removed the Win32 part of the name because it may be used by DisassemblerElf.
I created test cases for Rel32FinderX64 which extends those for Rel32FinderX86.

BUG=617965

Review-Url: https://codereview.chromium.org/2008253004
Cr-Commit-Position: refs/heads/master@{#398326}