powerpc/kasan: Fix stack overflow by increasing THREAD_SHIFT

When CONFIG_KASAN is selected, the stack usage is increased.

In the same way as x86 and arm64 architectures, increase
THREAD_SHIFT when CONFIG_KASAN is selected.

Fixes: 2edb16ef ("powerpc/32: Add KASAN support")
Reported-by:  <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=207129
Link: https://lore.kernel.org/r/2c50f3b1c9bbaa4217c9a98f3044bd2a36c46a4f.1586361277.git.christophe.leroy@c-s.fr