rust: Fix coverage in no-std crates

Coverage builds for no_std crates currently fail because
profiler_builtins are part of libstd and therefore do not get included.

When calculating the dependencies for coverage enabled targets, check if
the stdlib is being included, and if it is not then pull the
profiler_builtins sysroot.

Bug: 249551848
Test: CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS='*' make libtinyvec_nostd.rlib-std
Change-Id: I73f89bddcb2d4d9c704cb3962ee39b73888c7f3f