Add runtime id for Arch Linux

Add runtime id for Arch Linux

Like gentoo, Arch Linux is a rolling release and doesn't have explicit
version numbers.

    $ cat /etc/os-release
    NAME="Arch Linux"
    PRETTY_NAME="Arch Linux"
    ID=arch
    BUILD_ID=rolling
    ANSI_COLOR="0;36"
    HOME_URL="https://www.archlinux.org/"
    DOCUMENTATION_URL="https://wiki.archlinux.org/"
    SUPPORT_URL="https://bbs.archlinux.org/"
    BUG_REPORT_URL="https://bugs.archlinux.org/"
    LOGO=archlinux

Commit migrated from https://github.com/dotnet/corefx/commit/1f08b57bfd197a5492fe2ed0e0c5b2e6de9bf642