y2038: stat: avoid 'time_t' in 'struct stat'

The time_t definition may differ between user space and kernel space,
so replace time_t with an unambiguous 'long' for the mips and sparc.

The same structures also contain 'off_t', which has the same problem,
so replace that as well on those two architectures and powerpc.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>