[libFuzzer] Do not use llvm-objdump for disassembling a DSO.

When disassembling a DSO, for calls to functions from the PLT, llvm-objdump only
prints the offset from the PLT, like: <.plt+0x30>.

While objdump and dumpbin print the function name, like:
<__sanitizer_cov_trace_pc_guard@plt>

When analyzing the coverage in libFuzzer we dissasemble and look for the calls
to __sanitizer_cov_trace_pc_guard.

So, this fails when using llvm-objdump on a DSO.

Differential Revision: https://reviews.llvm.org/D29372

llvm-svn: 293791