y2038: Provide aliases for compat helpers

As part of the system call rework for 64-bit time_t, we are restructuring
the way that compat syscalls deal with 32-bit time_t, reusing the
implementation for 32-bit architectures. Christoph Hellwig suggested a
rename of the associated types and interfaces to avoid the confusing usage
of the 'compat' prefix for 32-bit architectures.

To prepare for doing that in linux-4.20, add a set of macros that allows to
convert subsystems separately to the new names and avoids some of the
nastier merge conflicts.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Christoph Hellwig <hch@lst.de>
Cc: y2038@lists.linaro.org
Cc: John Stultz <john.stultz@linaro.org>
Cc: Deepa Dinamani <deepa.kernel@gmail.com>
Link: https://lkml.kernel.org/r/20180821203329.2089473-1-arnd@arndb.de