tools: address possible non-null terminated filenames

If a filename is a multiple of 18 characters, there will be no null-terminator.
This will result in an invalid access by the constructed StringRef.  Add a test
case to exercise this and fix that handling.  Address this same vulnerability in
llvm-readobj as well.
tools: simplify symbol handling in objdump

Rather than switching behaviour on whether a previous symbol has an auxiliary
symbol record for the next count of elements, simply iterate over the auxiliary
symbols right after processing the current symbol entry.  This makes the
behaviour much simpler to follow and similar to llvm-readobj and yaml2obj.