Let ukernel plugin entry points alias the underlying ukernel (#13597)

Plugin entry points have their own signature, taking 3 pointer args.
They then call the actual ukernel entry point, passing it one of those
pointers.
https://github.com/openxla/iree/blob/2e939b0cb331ba07a62285c3268329394a665355/experimental/cpu_ukernel/plugin.c#L32-L37

Initially I chose to let the plugin advertise the plugin entry points
using a custom namespace, `"ukernel."`, to disambiguate from the
underlying entry point, namespaced `iree_uk_`.

But in #13460, @MaheshRavishankar is introducing bitcode ukernels, where
the true `iree_uk_` ukernel function is called directly. In that case,
the ukernel op needs to have that as its function-name attribute, and so
Mahesh has to change that there, but then that means that the resulting
compiled modules can't call plugins anymore. We would have to introduce
a new iree-compile command-line switch to tell whether to use only
bitcode ukernels or only plugin ukernels. There's no fundamental problem
with that, but it's nice if we can avoid the additional switch and have
everything just work.

Apparently, this works. My only concern with this is that now
`iree_uk_mmt4d` means 2 separate things, the actual ukernel and its
exported name in the plugins. I think that's fine: once bitcode lands,
plugins should be a minority case and for users who care about that
minority case, this will be a lot more convenient.