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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359878 91177308-0d34-0410-b5e6-96231b3b80d8