Add CentOS 8 runtime ids (dotnet/corefx#41397)

Add CentOS 8 runtime ids (dotnet/corefx#41397)

CentOS 8 has been released:
https://wiki.centos.org/Manuals/ReleaseNotes/CentOSLinux8

    $ cat /etc/os-release
    NAME="CentOS Linux"
    VERSION="8 (Core)"
    ID="centos"
    ID_LIKE="rhel fedora"
    VERSION_ID="8"
    PLATFORM_ID="platform:el8"
    PRETTY_NAME="CentOS Linux 8 (Core)"
    ANSI_COLOR="0;31"
    CPE_NAME="cpe:/o:centos:centos:8"
    HOME_URL="https://www.centos.org/"
    BUG_REPORT_URL="https://bugs.centos.org/"
    CENTOS_MANTISBT_PROJECT="CentOS-8"
    CENTOS_MANTISBT_PROJECT_VERSION="8"
    REDHAT_SUPPORT_PRODUCT="centos"
    REDHAT_SUPPORT_PRODUCT_VERSION="8"

Commit migrated from https://github.com/dotnet/corefx/commit/6f82bd59a05ea9556397596ae9e59f39b904b222