build_tools: fix: ensure that iree-flatcc-cli and iree-c-embed-data are build for the target during cross-compilation (#22755)

When running a cross-compilation of the runtime I noticed that the
install includes binaries from the host binary directory that I
specified, specifically: `iree-c-embed-data`. Looking at
`third-party/iree/build_tools/embed_data/CMakeLists.txt:8` you can see
that the binary is only imported but never built for the cross-target.

When trying to also cross-compile the python bindings the cmake setup
was failing with:
```
CMake Error at third-party/iree/runtime/bindings/python/CMakeLists.txt:334:EVAL:2 (install):
  install TARGETS given target "iree-c-embed-data" which does not exist.
Call Stack (most recent call first):
  third-party/iree/CMakeLists.txt:DEFERRED
```

This happens because the binary is not being build for the cross-target.
The same issue happens for the `iree-flatcc-cli` tool.

This PR ensures that the host binaries are correctly used during the
cross-compilation, as well as ensuring that the binaries are built for
the target and included in the install.

Signed-off-by: Florian Walbroel <walbroel@roofline.ai>
Co-authored-by: Florian Walbroel <walbroel@roofline.ai>