[mlir][bazel] Fix build after moving AMX into X86 in #183717. (#184165)
This PR fixes the bazel build that got broken by #183717, which moved
the AMX dialect into the X86 dialect. The fix consists of replicating
the changes from the CMake files into BUILD files as usual; in this
case, mostly removing the AMX dialect targets, adding a few new
references to the corresponding X86 targets, and adding a few new
dependencies to the existing X86 targets due to the new code.
Signed-off-by: Ingo Müller <ingomueller@google.com>