[Object][XCOFF] Add an XCOFF dumper for llvm-readobj.

Patch adds support for dumping of file headers with llvm-readobj. XCOFF
object files are added to test dumping a well formed file, and dumping
both negative timestamps and negative symbol counts, both of which are
allowed in the XCOFF definition.

Differential Revision: https://reviews.llvm.org/D60878

llvm-svn: 359878