Replace afl-clang-fast with gclang for non-instrumented builds

[zchengchen]

It seems that there is no need to modify the compiler to add the --notI parameter because afl-fast-clang is just a frontend for clang. If instrumentation is not needed, gclang can be used directly without affecting the extraction of bytecode.

Chaning:

export ADD="-g --notI "
export CC=/home/WAFLGo/afl-clang-fast CXX=/home/WAFLGo/afl-clang-fast++  CFLAGS="$ADD" CXXFLAGS="$ADD"
export AFL_CC=gclang AFL_CXX=gclang++

to:

export ADD="-g"
export CC=gclang CXX=gclang++  CFLAGS="$ADD" CXXFLAGS="$ADD"
export AFL_CC=gclang AFL_CXX=gclang++

does not seem to affect the functionality.