syscall: Add ioctl consts from if_tun.h
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
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@226196 138bc75d-0d04-0410-961f-82ee72b054a4