Add an alloy.py option to build ispc binary with g++

This will add an option '--ispc-compiler' to the 'alloy.py' which allows building ispc binary with g++ instead of clang. Possible options are: 'clang', 'g++', 'gcc', with 'g++' and 'gcc' being equal. Clang is still a default option. Also, 'check_isa.cpp' is now being compiled with g++, as clang may not be the option on every machine.