[llvm-objcopy] Refactor code to include initialize method

This change refactors some of the code to allow for some code
deduplication in later diffs as well as just to make adding a new
section type more self contained to the class itself. The idea for this
was first mentioned by James in D 37915 and will be used in that change
as recommended.

This change follows changes for dynamic sections but precedes support
for dynamic relocations.

Differential Revision: https://reviews.llvm.org/D38008

llvm-svn: 314148