Use @loader_path instead of $ORIGIN in rpath on macos. Issue #4480.

[alexanderlobov]

This is the same pull request as #4541, but with a different email address (due to some issues with CLA).

This patch fixes the issue with $ORIGIN in rpath on macos #4480. I'm not sure whether it is the best way to solve the issue, but it works :) I have a superficial knowledge about bazel's code, so feel free to correct me if I'm wrong in some way.

There a still several open questions:

1. I'm not use should we use @loader_path or @executable_path. As I understand, it is the same for plain executables, and only makes a difference for mac's frameworks or 'app' directories. But description in man dyld is pretty crypric to me.

2. There is a file tools/osx/crosstool/CROSSTOOL.tpl, that conains "-Wl,-rpath,$ORIGIN/%{runtime_library_search_directories}". I have tried to change it too, but it have not affected my test project. I'm not sure should I change this file too.

3. I tested as described here. If there is another way, say me.

   bazel build //:bazel-distfile
   #unzip to a tmp directory
   bazel test //src/... //third_party/ijar/...
   

I tested using this project.

[bazel-io]

Can one of the admins verify this patch?

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