Move --linkopt flags into user_link_flags

Previous behavior was to put flags coming from Bazel option --linkopt into
legacy_link_flags. They should be in user_link_flags instead (together with
flags coming from linkopts rule attribute). This cl introduces
--experimental_linkopts_in_user_link_flags option that flips the behavior.

There is another incompatible change. Previously cc_common.create_link_variables() included flags from --linkopt, with the flag flipped it doesn't anymore. I believe
--linkopt flags shouldn't be there by default because:
* We don't tie the API with the specifics of C++ rules/options, enabling theoretical use with other languages (objc)
* Users are free to use ctx.fragments.cpp to access C++ options and add them explicitly (https://github.com/bazelbuild/bazel/issues/5602)
* New behavior maintains the symmetry with --copt and user_compile_flags

RELNOTES: None.
PiperOrigin-RevId: 205274272

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