[Object][XCOFF] Add support for 64-bit file header and section header dumping.

Adds a readobj dumper for 32-bit and 64-bit section header tables, and extend
support for the file-header dumping to include 64-bit object files. Also
refactors the binary file parsing to be done in a helper function in an attempt
to cleanup error handeling.

Differential Revision: https://reviews.llvm.org/D63843

llvm-svn: 365524