Rust library linking fixes

- Hide universal binary dependency for rust

Without a darwin_arm64 libstd prebuilt, many rust binaries are not
possible to build as a universal binary anyway. This fixes an
intermediate error with compiling rust FFI targets, where C++ shared
library dependencies for both architectures were combined into the same
linker command.

There is still an outstanding problem with C++ executable targets that
include rust FFI library targets, as the rust FFI library targets
fail to find a proper libstd to build the arm64 side of the universal
executable.

- Use dynamic_lib instead of soname

This matches similar logic in cc/library.go : linkerFlags for including
shared libraries.

Bug: 291830024
Test: m libkmr_cf_ffi (when enabled for darwin)
Change-Id: I1d6b6a7855c6649b93ab6592ea43b17c6e37b3a3