[rust/coverage] Pass -z,nostart-stop-gc to the linker

Bug: http://b/199203503

Upstream LLVM change 6d2d3bd0a6 made -z,start-stop-gc the default.  It
drops metadata sections like __llvm_prf_data unless they are marked
SHF_GNU_RETAIN.  https://reviews.llvm.org/D97448 marks generated
sections, including __llvm_prf_data as SHF_GNU_RETAIN.  However this
change is not in the Rust toolchain.  Since we link Rust libs with new
lld, we should use nostart-stop-gc until the Rust toolchain updates past
D97448.

Test: m unicode-xid_device_test_src_lib and verify that they have the
__llvm_prf_data section.

Change-Id: I55eb3622dae9bd789fbacfe708600316102c365f