Compat mode (CONFIG_COMPAT) allows application binaries built for a
32bit architecture to issue system calls to a 64bit kernel which
implements such compatibility support.

This work builds on the y2038 sanitization which eliminated the
remaining assumptions about the size of native long integers in the
same move.  Although these changes enable armv7 binaries to call an
armv8 kernel, most of them are architecture-independent, and follow
this pattern:

- .compat_ioctl and .compat_oob_ioctl handlers are added to all file
  operations structs, pointing at compat_ptr_ioctl()
  compat_ptr_oob_ioctl() respectively.

- all pointers passed by applications within ioctl() argument structs
  are conveyed as generic 64bit values.

These changes mandate upgrading the ABI revision to #20.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>