powerpc: enable UBSAN support

This hooks up UBSAN support for PowerPC.

So far it's found some interesting cases where we don't properly sanitise
input to shifts, including one in our futex handling.  Nothing critical,
but interesting and worth fixing.

[valentinrothberg@gmail.com: arch/powerpc/Kconfig: fix typo in select statement]
Signed-off-by: Daniel Axtens <dja@axtens.net>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>