Move termios2 struct and ioctl constants to linux::arch::*.
When trying to use the newly added ioctls from #2508 I noticed that the `termios2` struct was not defined for musl or uclibc, except interestingly for the hexagon+musl target
However, the `termios2` struct and ioctls are part of the Linux headers, not part of the libc headers. So the libc flavour doesn't matter for these definitions.
This PR moves the definitions to the `linux::arch::*` modules, to make them available for all libc flavours and reduce code duplication as a bonus.
25ea72c5ce