selftests: vDSO: fix vdso_config for powerpc

selftests: vDSO: fix vdso_config for powerpc

Running vdso_test_correctness on powerpc64 gives the following warning:

  ~ # ./vdso_test_correctness
  Warning: failed to find clock_gettime64 in vDSO

This is because vdso_test_correctness was built with VDSO_32BIT defined.

__powerpc__ macro is defined on both powerpc32 and powerpc64 so
__powerpc64__ needs to be checked first in vdso_config.h

Fixes: 693f5ca08ca0 ("kselftest: Extend vDSO selftest")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>