powerpc/feature: Use CONFIG_PPC64 instead of __powerpc64__ to define possible features

In order to build VDSO32 for PPC64, we need to have CPU_FTRS_POSSIBLE
and CPU_FTRS_ALWAYS independant of whether we are building the
32 bits VDSO or the 64 bits VDSO.

Use #ifdef CONFIG_PPC64 instead of #ifdef __powerpc64__

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201126131006.2431205-1-mpe@ellerman.id.au