Linux: Define TIMER_T_WAS_INT_COMPAT in kernel-posix-timers.h

This is almost equivalent to __WORDSIZE == 64
&& OTHER_SHLIB_COMPAT (librt, GLIBC_2_1, GLIBC_2_3_3), except
that this expression is true for mips64/n64 targets as well,
even though those did not undergo the timer_t transition.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>