PECOFF: Set the AddressOfRelocationTable in the DOS header

Many programs, for reasons unknown, really like to look at the
AddressOfRelocationTable to determine whether or not they are looking at
a bona fide PE file.  Without this, programs like the UNIX `file'
utility will insist that they are looking at a MS DOS executable.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221335 91177308-0d34-0410-b5e6-96231b3b80d8