Pass -Wl,--as-needed when linking Rust binaries.

aosp/2518976 revealed a pre-existing bug, which was that some Rust test
targets did not have all of their shared library dependencies packaged in
the host-unit-tests.zip file. Conveniently enough, these targets did not
actually depend on any of the symbols in the missing shared libraries, so
their DT_NEEDED entries were being removed by the --as-needed flag being
passed to the linker by rustc. This flag was dropped when we switched
to running the link command directly, causing the DT_NEEDED entries to
reappear and leading to runtime failures for these test binaries.

Although we could fix the issue that prevented these (unused) dependencies
from being packaged, there wouldn't be much point in doing so because
we already know that they aren't needed. So for now, let's return to
the status quo and keep passing --as-needed to the linker.

Bug: 278168700
Change-Id: Iae103803983240fa14cdbc2ff0d8ffb4329c6024