JuliaC libraries should not install signal handlers

[topolarity]

In other Julia-as-a-library projects, we force single-threaded mode and disable signal-handler installation:

    jl_options.image_file = sysimage_path;
    jl_options.handle_signals = JL_OPTIONS_HANDLE_SIGNALS_OFF;
    jl_options.nthreadpools = 1; // We must only have one thread pool
    jl_options.nthreads = 1;

this is to play nice with executables that need these.

JuliaC.jl should almost certainly do the same (at least until it has multi-threading / chained signal support).