Disable the MLIR ExecutionEngine library when the native target is not configured

The execution engine would not be functional anyway, we're already
disabling the tests, this also disable the rest of the code.

Anecdotally this reduces the number of static library built when the
builtin target is disabled goes from 236 to 218.

Here is the complete list of LLVM targets built when running
`ninja check-mlir`:

libLLVMAggressiveInstCombine.a
libLLVMAnalysis.a
libLLVMAsmParser.a
libLLVMBinaryFormat.a
libLLVMBitReader.a
libLLVMBitstreamReader.a
libLLVMBitWriter.a
libLLVMCore.a
libLLVMDebugInfoCodeView.a
libLLVMDebugInfoDWARF.a
libLLVMDemangle.a
libLLVMFileCheck.a
libLLVMFrontendOpenMP.a
libLLVMInstCombine.a
libLLVMIRReader.a
libLLVMMC.a
libLLVMMCParser.a
libLLVMObject.a
libLLVMProfileData.a
libLLVMRemarks.a
libLLVMScalarOpts.a
libLLVMSupport.a
libLLVMTableGen.a
libLLVMTableGenGlobalISel.a
libLLVMTextAPI.a
libLLVMTransformUtils.a

Differential Revision: https://reviews.llvm.org/D117287