Move _tpu_ext.cc to jaxlib/mlir/_mlir_libs and set RPATH correctly
_tpu_ext.so dynamically links in libjaxlib_mlir_capi.so (in
jaxlib/mlir/_mlir_libs), so needs to include jaxlib/mlir/_mlir_libs in
its RPATH or similar on other platforms.
We achieve this by moving _tpu_ext.cc to jaxlib/mlir/_mlir_libs so it
can use the same linkopts as other mlir targets that depend on
libjaxlib_mlir_capi.so. In particular, we want this to work correctly
across platforms, and it's not clear if Windows supports RPATH-like
functionality beyond the current directory.
PiperOrigin-RevId: 551372130
