selftests: vDSO: fix vDSO name for powerpc

selftests: vDSO: fix vDSO name for powerpc

Following error occurs when running vdso_test_correctness on powerpc:

~ # ./vdso_test_correctness
[WARN]	failed to find vDSO
[SKIP]	No vDSO, so skipping clock_gettime() tests
[SKIP]	No vDSO, so skipping clock_gettime64() tests
[RUN]	Testing getcpu...
[OK]	CPU 0: syscall: cpu 0, node 0

On powerpc, vDSO is neither called linux-vdso.so.1 nor linux-gate.so.1
but linux-vdso32.so.1 or linux-vdso64.so.1.

Also search those two names before giving up.

Fixes: c7e5789b24d3 ("kselftest: Move test_vdso to the vDSO test suite")
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>