Fuzzers - explicit C compiler

[GrosQuildu]

Frontends specifies compiler for c++ code only:

deepstate/bin/deepstate/executors/fuzz/afl.py

Line 33 in 67e98f5

"COMPILER": "afl-clang++"

There are two issues related to that fact:

1. We strip "++" from the compiler to get compiler name for C code:

   deepstate/bin/deepstate/core/fuzz.py

   Lines 373 to 375 in 67e98f5

   	env["CC"] = self.compiler_exe.replace('++', '')
   	env["CXX"] = self.compiler_exe
   	L.debug("CC=%s and CXX=%s", env['CC'], env['CXX'])

   
   We should allow frontends to explicitly specify C compiler.

2. We use only C++ compiler anyway:

   deepstate/bin/deepstate/core/fuzz.py

   Lines 378 to 380 in 67e98f5

   	compiler_args: List[str] = ["-std=c++11", self.compile_test] + flags + ["-o", _out_bin] # type: ignore
   	compile_cmd = [self.compiler_exe] + compiler_args
   	L.debug("Compilation command: %s", compile_cmd)

   
   We should detect if compiled file is C or C++ and set the compiler accordingly. Unless env vars we set earlier in the code do the job?