Bazel: passing --embed-file to linkopts results in file not found

Synopsis:
```
cc_binary(
name = "embed-file",
srcs = ["embed-file.cc"],
linkopts = ["--embed-file", "$(rootpath my-file.txt)"],
data = ["my-file.txt"],
)
```
errors out on `file_packager` stating `%PATH%/my-file.txt does not exist`.

I have poked around and the path resolution looks correct, `--sandbox_debug` gives me some paths I can manually expect (namely `PWD=/proc/self/cwd`) where the path is present, but if I put `ls -lR` in `emcc_link.sh`, it's not visible there. Seems to me that the file is not passed to the toolchain - `data` merely exposes it in runfiles, and it cannot be included in `srcs`, etc.

@walkingeyerobot have you played around with this at all?