powerpc: make facility_unavailable_exception 64s

The facility unavailable exception is only available on ppc book3s
machines so use CONFIG_PPC_BOOK3S_64 rather than CONFIG_PPC64.
tm_unavailable is only called from facility_unavailable_exception so can
also be under this Kconfig symbol.

Signed-off-by: Rashmica Gupta <rashmica@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220617042805.426231-1-rashmica@linux.ibm.com