libgo: refactor code to enumerate stdlib packages

libgo: refactor code to enumerate stdlib packages

    Move the list of libgo, gotool, and check-target packages into
    separate files, then read the file contents as part of the build
    process on the fly. This is intended to enable other build tooling to
    share the canonical list of target packages (avoid duplication).

    Reviewed-on: https://go-review.googlesource.com/89515

    libgo: revise rules for runtime.inc generation

    Refactor code for generating runtime.inc: extract out the relevant
    commands and place them in a separate shell script ("mkruntimeinc.sh").
    Update rules to avoid generating macros whose names begin with "$",
    such as "#define $sinkconst0 0".

    Reviewed-on: https://go-review.googlesource.com/85955

From-SVN: r259863