[libFuzzer] Portably disassemble and find calls to sanitizer_cov_trace_pc_guard.

Instead of directly using objdump, which is not present on Windows, we consider
different tools depending on the platform.
For Windows, we consider dumpbin and llvm-objdump.

Differential Revision: https://reviews.llvm.org/D28635

llvm-svn: 292739