pps: Compatibility hack should be X86-specific

As of [1], using PPS_FETCH on a 64-bit ARM kernel with a 32-bit userland
is broken, returning a timeout. This is because the requested 4-byte
alignment for struct pps_ktime_compat (illegal on arm64) results in the
timeout flags field being uninitialised.

Make the hack specific to X86_64 builds with CONFIG_COMPAT defined.

[1] commit c2a49fe8 ("pps: fix padding issue with PPS_FETCH for
    ioctl_compat")

See: https://github.com/raspberrypi/linux/issues/5430
Fixes: c2a49fe8

 ("pps: fix padding issue with PPS_FETCH for ioctl_compat")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>