powerpc/32: Add VDSO version of getcpu on non SMP

Commit 18ad51dd ("powerpc: Add VDSO version of getcpu") added
getcpu() for PPC64 only, by making use of a user readable general
purpose SPR.

PPC32 doesn't have any such SPR.

For non SMP, just return CPU id 0 from the VDSO directly.
PPC32 doesn't support CONFIG_NUMA so NUMA node is always 0.

Before the patch, vdsotest reported:
getcpu: syscall: 1572 nsec/call
getcpu:    libc: 1787 nsec/call
getcpu:    vdso: not tested

Now, vdsotest reports:
getcpu: syscall: 1582 nsec/call
getcpu:    libc: 502 nsec/call
getcpu:    vdso: 187 nsec/call

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eaac4b6494ecff1811220fccc895bf282aab884a.1575273217.git.christophe.leroy@c-s.fr