Set the RPATH of _portable_lib.so so it can find libtorch

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pip wheels will need to be able to find the torch libraries. On Linux, the .so has non-absolute dependencies on libs like "libtorch.so" without paths; as long as we import torch first, those dependencies will work.

But Apple dylibs do not support non-absolute dependencies, so we need to tell the loader where to look for its libraries. The LC_LOAD_DYLIB entries for the torch libraries will look like "@rpath/libtorch.dylib", so we can add an LC_RPATH entry to look in a directory relative to the installed location of our _portable_lib.so file.

To see these LC_ values, run `otool -l _portable_lib.so`.

Differential Revision: D56857492

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@dbort merged this pull request in pytorch/executorch@da2d695dce8ddaea198ac0c80ca60374f0c09e4a.