sys_linux: fix building with older compilers and some archs

sys_linux: fix building with older compilers and some archs

The recent replacement of <termios.h> with <linux/termios.h> to get
TCGETS2 seems to work only with compilers (or C standards) that allow
the same structure to be defined multiple times. There is a conflict
between <sys/ioctl.h> and <linux/termios.h>.

Another problem is that TCGETS2 is not used on some archs like ppc64.

Switch back to <termios.h> and move TCGETS2 to a list in a separate
file where it can be compiled without <sys/ioctl.h>.

Fixes: 03875f1ea5c4 ("sys_linux: allow ioctl(TCGETS2) in seccomp filter")