twister: Set a default gcov-tool for llvm

When using llvm we need to set the gcov-tool to "llvm-cov gcov" but
the lcov tool is incapable of passing arguments to the gcov-tool. i.e.
the following cannot work:

$ lcov --gcov-tool "llvm-cov gcov" ...

Instead, create a symlink to llvm-cov prefixed as `gcov` which by the
documentation of llvm-cov will alias to `llvm-cov gcov` subcommand.

Signed-off-by: Yuval Peress <peress@google.com>