crtsavres.S content is encloded by a #ifndef CONFIG_PPC64

To be used on VDSO32 on PPC64 it's content must available on PPC64 as
well.

Replace #ifndef CONFIG_PPC64 by #ifndef __powerpc64__ as __powerpc64__
is not set when building VDSO32 on PPC64.
Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Closes: https://lore.kernel.org/linuxppc-dev/047b7503-af0c-4bb0-b12a-2f6b1e461752@csgroup.eu/T/
Fixes: b163596a ("powerpc/vdso32: Add crtsavres")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/aded2b257018fe654db759fdfa4ab1a0b5426b1b.1726772140.git.christophe.leroy@csgroup.eu