The "platform" flag: add support files from lfc automatically and debug the spike platform

[lsk567]

This feature should enable lfc to automatically pull in the right support files during code generation.
Take the following program for example.

target C {
    platform: spike
}

main reactor {
    reaction(startup) {=
        printf("Hello world\n");
    =}
}

After code generation, a correct platform support file for spike should appear in src-gen and can be cross-compiled to RISCV with the rest of the c files.