rust: Disable dead stripping for fuzzers.

Without `-C link-dead-code`, Rust fuzzers may fail to start fuzzing.
See the cargo-fuzz issue in the code comment for details. This patch
disables dead stripping for our fuzzers by default.

Bug: 193200135
Test: Flag emitted when building Rust fuzzers.
Test: Fuzzer runs on device.
Change-Id: I530de30e9a57db206bcbf3d2d3d371628eef8934