rust: Use host linker when building for Mac host.

This behavior is non-hermetic, but matches the behavior of the C++
support. On Linux, using lld works, but on OSX, this fails because lld
does not support Apple's new .tbd files.

Bug: 155302034
Test: On both Linux and Mac hosts:
    cd external/rust; mma

Change-Id: I0ad489113d720bdb9c3b7a67cce9d1e72266f428