Don't export link dirs from proc-macros.

We shouldn't use exported link dirs from proc-macros as these may be
libraries for the wrong architecture or OS.

This doesn't cause a problem unless both the proc-macro and the
dependent library share a dependency. If that's the case, in some
circumstances it will cause the depdent library to fail compilation
when using a library with incorrect architecture.

Bug: 163088025
Test: Test case no longer colliding.
Test: cd external/rust/; mma
Change-Id: I46cbd850c3a89d10aa844b5c1b383e1070452d00