During the gccgo build define constants from
    some header files are processed by mksysinfo.sh and
    added to syscall.go.  In some cases the constants
    are based on macros which are not processed
    correctly by mksysinfo.sh.  This adds the support
    to generate the correct values for the TUN* constants
    from linux/if_tun.h.
    
    Fixes golang/go#11707
    
    Reviewed-on: https://go-review.googlesource.com/12491

From-SVN: r226196