Driver: only set -mlinker-version based on host if it's valid

We were adding an extra "-mlinker-version" argument to the invocation
based on a value inferred from "ld -v". This is set by the build
systems to either a sane value or an empty string (e.g. for custom
built ld), which we don't want to pass on.

No test really possible because the value depends on both host system
and how CMake was invoked.

llvm-svn: 239633