[MLIR] Fix checks for native arch

Using if (TARGET ${LLVM_NATIVE_ARCH}) only works if MLIR is built
together with LLVM, but not for standalone builds of MLIR. The
correct way to check this is
if (${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_TO_BUILD), as the
LLVM build system exports LLVM_TARGETS_TO_BUILD.

To avoid repeating the same check many times, add a
MLIR_ENABLE_EXECUTION_ENGINE variable.

Differential Revision: https://reviews.llvm.org/D131071