The darwin linker allows setting the LTO library with the -lto_library
flag. This wasn't caught by our "safe linker flags" check because it
was covered by the -lx flag used for linking libraries. This change
adds a specific check for excluded flags which otherwise satisfy our
existing checks.
Loading a mallicious LTO library would allow an attacker to cause the
linker to execute abritrary code when "go build" was called.