[funfuzz] Consider moving to compile fully using Clang on Linux

Currently, we sometimes/mostly use GCC for compiling on Linux. [Bug 1253064](https://bugzilla.mozilla.org/show_bug.cgi?id=1253064) shifts the main Mozilla development one to Clang, and I think we hardcode in GCC only for some scenarios, we should sort out our Clang building story since we have a `--build-with-clang` option that we might want to remove as it is now the default.