rust: Do not pass -C prefer-dynamic for cdylib

cdylib targets cannot currently dynamically link to Rust crates, so this
flag was not actually doing anything, and in conjunction with -C lto
which we *do* want on for this target, causes a hard error on
rustc-1.40.0.

Bug: 147438124
Test: Build a rust_library module, this implicitly tries to build cdylib
Change-Id: I828b3e6bae7c58f4081db3e73009b443a382a296