Using gcc for instrumentation

Many projects do not support clang. It seems that fuzztest uses `-fsanitize=address,-fsanitize-coverage=inline-8bit-counters,-fsanitize-coverage=trace-cmp` to compile the target program with coverage information.

Would it be possible to use gcc-based instrumentation? For example, in the same fashion as [AFL++ instrumentators do](https://github.com/AFLplusplus/AFLplusplus/blob/stable/docs/fuzzing_in_depth.md#1-instrumenting-the-target)?