llvm-objdump: initial man page
Based on llvm-objdump's online documentation and usage information.
This serves as a starting point; additional detail and cleanup still
Also being submitted upstream in LLVM review D54864. I expect to use
this bespoke copy while we have LLVM 6.0 or 7.0 in FreeBSD; when we
update to LLVM 8.0 it should be upstream and we will switch to it.