[mlir] Tunnel LLVM_USE_LINKER through to the standalone example build.

When building in debug mode, the link time of the standalone sample is excessive, taking upwards of a minute if using BFD. This at least allows lld to be used if the main invocation was configured that way. On my machine, this gets a standalone test that requires a relink to run in ~13s for Debug mode. This is still a lot, but better than it was. I think we may want to do something about this test: it adds a lot of latency to a normal compile/test cycle and requires a bunch of arg fiddling to exclude.

I think we may end up wanting a `check-mlir-heavy` target that can be used just prior to submit, and then make `check-mlir` just run unit/lite tests. More just thoughts for the future (none of that is done here).

Reviewed By: bondhugula, mehdi_amini

Differential Revision: https://reviews.llvm.org/D126585