MIPS: compat: Implement is_compat_task() by testing for 32-bit address space.

So far is_compat_task() was testing for 32-bit registers if O32 support
was enabled and if O32 support was disabled but N32 enabled it was testing
for 32-bit address space.  So if both O32 and N32 were enabled a N32
task was not considered a compat task, whops.

This still leaves potential cases where O32 and N32 need different treatment
unsolved.  But that's another commit.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>