Enable dynamic library measurement in dispatcher

I need to add 7z(binary 7z, with an instrumented 7z.so) as a new benchmark, in order to keep the testing manner as close as possible to fuzztrusion, I didn't statically link 7z.so.
which means in the dispatcher, I need to run
```
llvm-cov export ./fuzzer_7z -instr-profile=default.profdata -object=./7z.so -object=./fuzzer_7z > cov.json
```
instead of
```
llvm-cov export ./fuzzer_7z -instr-profile=default.profdata > cov.json
```
right now I use an self-defined option cov_objects in benchmarks/7zip_7z/benchmark.yaml to solve this problem