y2038: powerpc: Extend sysvipc data structures

powerpc, uses a nonstandard variation of the generic sysvipc
data structures, intended to have the padding moved around
so it can deal with big-endian 32-bit user space that has
64-bit time_t.

powerpc has the same definition as parisc and sparc, but now also
supports little-endian mode, which is now wrong because the
padding is made for big-endian user space.

This takes just take the same approach here that we have for
the asm-generic headers and adds separate 32-bit fields for the
upper halves of the timestamps, to let libc deal with the mess
in user space.

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