Adding libc++ explicitly to the shared libraries of rust-only tests

There is an issue with the boringssl build configuration right now, where
libc++ is not made available to a rust-only client. This CL adds an explicit
link to libc++.

Test: m checkbuild
Bug: 340583190
Change-Id: I8040f398e89bce793844fba6e75986ac95bfd625