[MLIR] Fix build without native arch

D125214 split off a MLIRExecutionEngineUtils library that is used
by MLIRGPUTransforms. However, currently the entire ExecutionEngine
directory is skipped if the LLVM_NATIVE_ARCH target is not available.

Move the check for LLVM_NATIVE_ARCH, such that MLIRExecutionEngineUtils
always gets built, and only the JIT-related libraries are omitted
without native arch.

Differential Revision: https://reviews.llvm.org/D125357