Always link the MLIR python bindings native extension to libMLIR.so

The Python bindings now require -DLLVM_BUILD_LLVM_DYLIB=ON to build.
This change is needed to be able to build multiple Python native
extension without having each of them embedding a copy of MLIR, which
would make them incompatible with each other. Instead they should all
link to the same copy of MLIR.

Differential Revision: https://reviews.llvm.org/D90813