timerfd: Relocate 32-bit compat code

32-bit compatibility code is conventionally stored in
sys/compat/freebsd32. Move freebsd32_timerfd_gettime() and
freebsd32_timerfd_settime() from sys/kern/sys_timerfd.c to
sys/compat/freebsd32/freebsd32_misc.c.

MFC After: 3 days
Reviewed by: imp, markj
Differential Revision; https://reviews.freebsd.org/D41640